Hello! Welcome to the Forum, and to event world. You’ve come to the right place, there are lots of eventers here with OTTBs who can be able to help you. I’ll reply with my insights, but would be remiss in mentioning a few things: 1. go to the track with an experienced OTTB retrainer - someone who has BTDT and has developed an eye for sussing out lamenesses and structural disparities that an average person might miss – 2. make sure you have a trainer who is willing to work with you with an OTTB!
You mentioned mid-UL prospect; I’ll assume that means something like Training/Prelim? Generally, most TBs can jump those heights - it’s the dressage or the soundness that might prevent them from that. I’ve picked up several OTTBs now and have developed my own preference for what I want to see in an OTTB.
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I want a great walk. I don’t want short steps, no overtrack, gait asymmetry or too fast/too slow. I want to see a loose, fluid, rolling walk - have you ever seen a lioness walk? that is the type of walk I envision. A good walk almost always correlates to a good gallop, a great walk to a great gallop – and one of the most important gaits for an event horse is the gallop.
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I keep my guys at home, so there are some lamenesses I am willing to overlook because I can give them the winter off. There is a difference between ‘tracky’ (which is body-soreness) and lame - I’ll take a chance on a tracky horse if all else is okay; but I will pass on a horse where I see obvious gait asymmetry behind.
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Conformation - this is the sum of all parts - how the horse moves, etc. I will let some flaws slide if the horse has 30-50+ starts and is a good mover - but some things I will not touch: straight stifle-to hock with short upright femur. Will not touch it. IME asking for sore stifles and suspensory issues. Some straightness behind is okay, but not the two together and not straightness from stifle to hock. Some straightness is okay if they have perfect hind end configuration otherwise (see Wild Brassy, Economic Forecast in links at bottom). Long weak loin with moderate straight hind legs - will not touch. Goose-rumped by itself is ok, not ok when combined with long loin or straight hinds. Overly long pasterns or toeing out, will not touch. Toeing in is okay, in most cases. Bucked shins + over at the knee, ok - over@ the knee is usually work-load induced and not actually a conformation flaw. I want to see a shoulder with a moderately set back elbow, long forearm and shortish cannon - this directly relates to how easily they can open up their stride, and usually correlates to good movement. I do not like horses that have very forward placed elbows that sit right on top of their shoulder. Usually combined with straighter shoulder as well.
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My ‘type’ - The first thing I look at is the type of ground the horse stands over. I want straight front legs, a sloped shoulder, a strong loin connection and I want to see a steep, long angle in the femur with a forward, almost high placed stifle, broad angled hocks, and a good amount of angle behind. I want to see a big, broad engine behind - with a forward LS placement near or over the point of hip. IME this speaks well of athleticism and ability to fold limbs over fences. The neck I’d prefer to come up and out but many race horses have lower set necks; I have a lot of flexibility when it comes to the neck, but next to no flexibility when it comes to the configuration of the haunches.
I will respond with my personal insights, but it is worth what you paid for it – take what works and leave the rest 
Comisario:
Really nice pedigree. Unbridleds Song horses make good mid-UL horses. Usually good movers. Usually longer in the back than I like. He’s got some good blood for eventing; Siphon, Deputy Minister and Cox’s Ridge… He’s going to be athletic. Off of the pedigree alone I like him.
Conformationally, he’d be a pass for me. I saw something in that LH that could be risky. He’s a little too long in the back for my preferences. He looks really sweet otherwise, and I hope he gets a good home where they give him groceries… he looks fit but I think he needs a lot of attention in the nutrition department. I think he’d make a fine eventer if you could isolate what is going on… and he’s a good mover.
First Choice:
I love his pedigree. You would have a lot of athleticism in this pedigree. I would expect a great canter, good movement. Linebred to Fappiano, proven event sire - and iMHO, I love First Samurai, but I might be biased. GC throws some very nice horses and passes along a lot of very nice dressage movement. Damside is good too, with some notable names for durability.
His conformation is much more ideal for me; if you took his front half and gave it to Comisario, you’d have almost my ideal type. He has a great shoulder, excellent forearm to cannon ratio, he’s got a nice defined neck, and good heartgirth, depth of barrel, and a better loin connection than the horse above. He is not without his flaws; I do not like how upright his pasterns look, he has an osselet up front, and needs some serious farrier attention. In his older pictures he does not look so butt-high, and I don’t like how in every picture the RH is not flexed/fully weight bearing. He does not have the amount of angle behind I prefer. I really like his type from the barrel forward, but would pass because I don’t love that hind end configuration or how he is walking/trotting on the RH. He looks like a character though, and if pedigree is any indication, would be a very smart, sharp horse for someone.
Forester’s Hope:
Pedigree: This mare, of all of them, probably has the most innate jump talent if she lives up to her pedigree. lots of good names, Forestry has some really good horses on the ground, Irish Castle/River are great turf horses, and her dam-side has some very good old blood up close - I’d expect eventing would be easy for her. I would not expect dressage type movement but would expect a good canter/gallop.
Conformationally it’s hard for me to assess her as her pix are not straight on - same problem as before though: I like her front end, don’t care for her hind end. She’s goose-rumped and I want to see what her actual hq looks like but can’t because of the angle… She looks cowhocked… not a deal breaker for me if they move squarely. in the video it doesn’t look terrible. I watched the video and of the three, she’d be the one I picked. She’s tense in the video but that has the making of a very, very nice trot and canter. That part where she got tense at the trot I saw a bobble on her RH but IMHO, I wonder if it is a strength issue. I found a newer video and the bobble wasn’t there.
Of the three, I think Forester’s Hope wins by a landslide, at least for me.
For an example of my ideal type, here are some I pulled from the FLF listings:
Little Gidding
Wild Brassy
Economic Forecast ** this guy was rescued by a COTHER and is looking for his forever home…
Midnight Tucker - the guy I took home
I noticed they were all bay… oops. Not on purpose, but I do like me a solid bay 
