Your best bet IMO to have it "maintain the cost you pay for it, when you go to sell it" is to get an already restored vintage trailer. If it is already restored "well" - either professionally or throughly documented - the seller probably put more $$$ into it than the asking price reflects. Assuming it is well maintained by you as owner, when you go to sell it it won't take as big of a hit as if you had bought a new one that lost value every year or bought newish one before the depreciation has been fully achieved. If you are the one buying cheap and restoring it...as others have said, you aren't likely to get your money back out.
I guess it's a matter of saving or spending $$$ vs having "exactly what you want". We have gone the route of getting exactly what we want - knowing full well we won't re-coup our entire cost of restoration and not regretted it. But after two resto's in 8 years, I would think very hard about doing another myself...maybe it's like childbirth, the further it gets behind you the less painful it is to remember.
Shari