1 thats long enough to fit
New or used
I can buy a brand new (but basic) bunk trailer for a 15ft aluminum runablut for about $700 +fees. Or buy a junk boat on a trailer for $1-300 then scrap the boat for about $40. Once the boat is gone, I can spend several weekends and $3-500 rehabbing the trailer.
Unless I can strip other needed parts (or motor) off the junk boat, I dont buy them anymore just to get the trailer. Too much work for the marginal cost savings vs new. I'll buy new when the time comes.
How long is the boat? Most trailer places will sell trailers that will fit a range of boat lengths. The 1 for my 15ftr I mention above, will fit 14-16ftrs.
Needs to be long enough to have a couple feet of tongue past the winch post and have the transom supported by the bunks.