I always rattle a door to see if it's open, so to speak. I also stay away from 73' and later bikes as the title thing can be a pain. I've bought 3 or 4 Triumphs for well under 1K, and I nailed a 500 Triumph Metisse for $1200. My philosophy is go and look. I'd give the guy a call. If he knows the bike is valuable, he knows the bike is valuable. You know what you've got to spend, nothing ventured, nothing gained.
Earlier this year I scored a real nice Triumph T100C for $1500, and then the guy gave me a Grumph frame (!). The engine needed rebuilding so I'm in it now for like $2200, probably worth $3500 easy, and I love it. I've gotten screwed a few times, but less and less as I get older.
The 350 Ducati's were probably the best of the single Duc' breed. Look at Pete T's, the thing will whip bikes that have 150cc's on it. Remember though, if the engine is toast, you've got a fairly expensive rebuild on your hands, so consider the condition of the motorcycle also when buying. If it's shitty, is it worth doing? I traded the guy back on that 250 Ducati after Pete pulled the top end and found it was not as represented. Fortunately, the guy was upstanding, but I still lost the money I paid Pete, and all the hassle and running around. If the rest of the bike was in decent shape, I'd have rebuilt it, but it needed too much.
Really, if they need work, unless it's a bike that you really want (and listen, I have happily taken that road to get something I very much wanted to own several times), buy it right. It's like buying an old house, there's always more to be done than meets the eye. And will it appreciate enough down the road when its mystique begins to lose its hold on you and you want to move it on? Many bikes will, many bikes won't.
When it comes to Japanese bikes, I only buy what is all there, runs well, and is in fairly good (at least) condition; this because the market is still fairly weak on them, and while some models do appreciate, not nearly enough to match resto costs. A guy had me up to his house the other day to look at a 305 Super Hawk "all there!". He wanted $500. What a roach, seized up with 35K on the clock. You could easily sink 5 to 6K in it, but you can buy a very nice one for $3500.