ooooh, risky. If he thinks it has gold embeded in it, leave it there for a few more years. If it's been sitting a while, first thing that is likely to happen once you get it rolling is the dry and old fork seals will blow which is a job I hate doing personally, and worse still if the stantions have pitted at all. All cables may need replacing or if you're lucky, a good lubing if they ain't about to ping. Brakes will likely need a good service as a must, all fluids will need changing, tyres may of started to crack around the edges, so....
They are just some little things off the top of my slightly cloudy minded head right now, but if you're going to make a cash offer, I would say I have seen runners going for around £350-400, also needing some TLC but runners none-the-less. Being bit of a potcher, I'd be pleased if I could get it for the £150-200 mark (maybe a bit generous), but these bikes aren't that sought after these days and original parts slightly more awkward to obtain.
This is just my opinion mind you and others may have different views, but I will say that I was tempted to buy a non-runner, however, complete for £75-100 about three months back - didn't have a trailer to collect though.
Just have a quick squint at the availability of parts you may need and the costs. May be a good indicator as to why the guy never bothered repairing it!!
Good luck.
