Steel 2H BP trailers can be purchased for relatively little money. Most can be bought outright for what you quoted the repair costs. A steel trailer with a rusted frame is not a bargain at any price. After sitting for that length of time, you will need wheel bearings, tires, brakes and electrical components in addition to the rust issues.

I would look else where for a trailer in better shape, and one that has been maintained. You may pay more in its purchase price, but spend far less to get it safely on the road. Once there, your goals are reliability, something hard to find with a rust issue.