Suggest buying the book by Neve Scheve about Buying, Owning, and Maintaining a Horse Trailer. She covers all your questions there and a lot more. https://www.amazon.com/Complete-Maintaining-Servicing-Trailer-Reference/dp/0876056869
I live in the "trailer capital of the world (Elkhart IN) where national brands are manufactured within 10 miles of my house: Lakota, Merhow, Bison, and about every LQ is “remanufactured” here to specifications.
I prefer Aluminum skin --but in 55 years, have yet to see an all aluminum fail or have a horse “kick through the side” as is often related.
Shiny aluminum exteriors will not affect the interior temperature. However, colored paint will —one manufacturer told me (when I asked about a black trailer to match my black truck) that every square foot of black paint on a trailer increases the temperature by 1 degree inside. So I stick with white paint --my new trailer (Merhow 2021) has aluminum up to the windows --I think it keeps horse from scratching paint with halter, rope, or whatever. I clean it by doing a 50/50 spray with vinegar and water. Acid wash or anything similar will look good for awhile, then you are back to whatever . . .enjoy the beauty of the aluminum --mine is “brushed” not shiny --I think most are that way.
Slant or straight. I prefer straight, my kids prefer slant.
Ramp or step up —Merhow charges MORE these days for a step up than a ramp --I liked my old (2004) step up because I only had to open TWO doors to get horse out --with my ramp, we’re talking two doors and a ramp to deal with.
Favorite are Hart, 4 Star, Platinum, Featherlite and MERHOW (I like them best because their plant is only 7 miles from my house --really nice people there) --avoid --well, don’t want to trash talk anyone --if you want a PM --I’ll dish. BUT the “tell” on horse trailers is the weld --look for neatly done welding on all places where metal meets metal --the “cheaper” trailers don’t weld, just bolt on. And watch for trim on fenders --again cheaper trailers don’t do it --and horse tied to a trailer with a raw (sharp) edge by his front feed is a recipe for disaster (seen it too many times) --you can put it on yourself after market, fyi –
Upgrades: Step up into the tack room (now an option on the Merhow --can’t tell you how many “little stools” I’ve smashed by driving over or fallen off taking a saddle out of the tack. AND lined (skinned) tack room --I like hanging shelving in my tack room (RubberMaid) AND I have windows that open in the front of my tack room, and a window that opens to the horse compartment from the tack room --I can open those and as I drive, air blows back on the horse and out the back windows.
Oh, and hang your spare tire on the driver’s side of the trailer --that way the horse (usually tied on the passenger side around here) won’t fool with it or the cover on it. And it won’t be in your way in the tack room.
Other upgrades --grommets (those do-dads you tie horse to) have two extra put up high so you can hang hay from them for horse while on the trailer, and of course two on each side (most come with one) so horses can be on the same side.
I like tack door on Passenger side --now offered and seem standard on the driver’s side --up to you.
As far as drop down v bus windows —I like bus —it’s a choice.