Tad Coffin. Remember that these run a little small, so this is one time you really should insist on an 18" and not compromise for the 17.5" that seems like a deal.
Besides having a flat seat, I think you need to ask yourself if you want the balance of the saddle to be forward, such that you sit right up by the pommel. I like to ride there. So I look at the shape of the seat not only in terms of depth, but where (front to back) the deepest point is. I also look at how much like a box or a U the seat is. In other words, is the deepest point pretty long? Or is it short such that you have just one place to set?
Just looking for a shallow seated saddle usually means you won’t find a saddle with a seat that locks you into one position. But that doesn’t mean that the converse is true: That all shallow seated saddles will let you sit up by the pommel if that’s what you like.
So if the TC is a saddle with a shallow seat that lets you sit up front, I find the County Stabilizer to be one that puts me farther back in a way I don’t like. Others will tell you differently. They probably have longer femurs than I do. But that’s another famously-shallow saddle.
Hope this helps.