Have them put a torsion bar axle on it and ditch the springs. Less to break, more durable, and rides better.
Trailer really only needs big enough to sit under the flat part of the raft. Since you are going with a flatbed that will be lower to the ground, it’s kind of up to you with how much hangover is ok. Lower is going to get hit with more road junk but you are using a small car too, only you know what kind of roads you’re primarily on (paved, gravel, rough, etc) Something else to think about is extra room for a shuttle bike/scooter, tongue box, etc. I’m pretty happy that my raft sits higher with storage underneath but if weight is a big issue then you might not want that.