the re-do needs of your trailer do sound expensive, people have already commented on the high cost of foam, but then good foam the firmness you want is well worth a good night's sleep. Our last foam replacement was for a full size piece for the larger bed in the back of our Burro
(we keep it in bed mode permanently) and it was $200 for an almost double bed size, but we bought it a bit too firm so as suggested, be sure you get what you need to be comfy. Less firm would have been less expensive. Getting the foam upholstered by someone is also expensive. Our bench bed mattress cost $125 12 years ago, that was foam and fabric we bought and the labor for someone else to do the upholstery. Shop around and find a small uphosltery shop who has good prices.
We made a new table top for our Burro
back when it was new-to-us out of a thick scrap of good quality plywood we had that had a nice maple finished surface, I sanded and sealed it and kept the natural wood look. If you do not have 'scrap' materials around to use you might be able to buy one good piece of finished plywood for both the counter top and table.
I suggest you search the forum and find those who have had to redo floors and floor supports and contact them for suggestions. A lot of those who have done major renovations have the skills, a shop to work in, and materials around from other projects, cost has a lot to do with what you can do yourself and what material you might have around to use.
With these small trailers it is possible to buy scrap remenets of flooring, countertop pieces, carpeting, or whatever you want or need at flooring and counter top places.
Good luck in fixing up and 'customizing' your egg!