While this website:
Price vs Condition deals primarily with Airstream values, take a look at "Table C" towards the bottom of the page...there is lots of good info on replacement part costs for budgeting puposes.
Another great resource is
Vintage Trailer Supply they are kind of the "one-stop-shop" for all things vintage trailer.
Anytime you can buy a vintage trailer for $500 it's going to be a "good deal" but expect to put several thousand into to it to make it road worthy & someplace you'd like to stay in. Without knowing the starting condition, it's pretty tough to say how much it will need...alot also depends on your expectations - do you just want something to keep you dry while camping or do you want it decked out as a showpiece?
Good luck and post pics, we all like pics!
Shari