The main things I understand about the trailers (we have an Excella, too, but much older and gutted) is to check the flooring, especially around the bathroom area (where water would possibly go) and to check the axles (you can search on this to find lots of good info). Checking the frame is also a good thing to do--check the belly pan for any gaps or holes, and if you have rotted wood above, you may want to check the frame below it, too, if you can. I am not sure how to otherwise check the frame, but do a search on it and I am sure a lot will come up.
I personally would be happier with a larger floor plan (bigger rather than smaller trailer), but you also need to consider the roads and accessibility for long vehicles in your area. If an 18-wheeler can get through, you and your trailer should fit just fine.
Good luck in your search! I hope you find your Dream 'Stream soon!