sub £3k isn't going to get you a very good whole package.
If you consider you need a trailer for the road that is roadworthy, an engine of at least 55hp plus (ideally 75-90) that works, and a hull and tubes that isn't holed or needs re-tubing, it will be over £3.5k IMHO.
If you can give on one of the three, ie you don't need a decent roadworthy trailer, you don't mind scrappy patched tubes, or don't mind having a poorer make/lower hp/old 2 smoke engine, then you may find something. Could be false economy though.
But if you want all three and also what everyone wants, a roadworthy roller
trailer, with non patched
/airtight tubes, undamaged hull (bar a few scrapes) and a Yam 2S
or modern 2Stroke/4stroke with PTT 75-90 , it will be >£4.5k.