Congratulations on finding a trailer that meets your needs... That's definitely a start in the right direction
As for the yellow streaks, most manufacturers use a "wood putty" material to fill the staple/nail holes in their trailers. On ours, we washed for several weeks (off and on) as we found the smudges which seem to only show up when the light hits the wall/ceiling at a certain angle. I used spray paint, a paper cup and a small artist brush to paint the holes to match the wall once we cleaned the putty off. What you're seeing is probably the "old discolored putty" but check very VERY closely to make sure there's not any water stain that has discolored.
Many RV's of all manufacturers had problems with the vinyl cracking in cold, freezing weather. If you can get the dealer or owner to replace it without increasing the price, that would be great, but if you can't, it shouldn't be a deal breaker if everything else is OK. There are a lot of good quality self adhesive tile products that mimic everything from slate to hardwood that would be perfect and not too hard to install for an average DIY'er.
As for the heat ducts, we've got 3 in our 28' fifth wheel, I'm going to install another one to get heat into the bath, but that's a project for when it warms up. Do a quick check and you may find that there are a couple of wall mounted 4" round heat outlets in the bedroom possibly mounted on the bed pedestal and maybe one in the bathroom as well. The floorplan you're looking at is pretty open, so there probably aren't a lot of heat outlets needed.
Good luck, hopefully, this may get you started on answers for your questions.