Corrosion can indeed be an issue; very important to make sure the the tanks have been out recently to enable inspection of the main spar, as per the ad. Ask to see proof of this in the logbooks.
Also worth looking back through G-INFO at the registration(s) and then googling them, to see if they come up in any AAIB reports. One a/c I looked at came up as having sustained major damage in a gear-up landing, which didn't show up in the logs (how did it get repaired and by whom) and appeared to be news to the owner.
As you rightly say, most a/c you see are financial time bombs with deferred problems the owners hope to pass on. Top money for an all singing and dancing Arrow II with low hours a/f, newish engine, Prop, decent paint interior and avionics seems to be £55k and for a similar spec Arrow III £65k. You can then work backwards from these figures (£15k for engine, £7-8k for paint, £2-3k for leather interior, £10-12k avionics suite, £5k for total undercarriage rebuild, £3-5k for new 3-blade hartzell or 2-blade McCauley).
In practice, what you find yourself being offered are A/C circa £50k needing £20k of work. There are several Arrow II's out there that have been for sale for 2 years or more at £35-40k, which need everything doing. Hence they've been for sale for 2 years.....
The degree of negotiation available will depend on how far out the initial price is. I think a blanket expectation that "all prices are negotiable by 20-30%" is not always realistic and will simply depend on the pricing of an individual a/c and the desperation of an owner to sell. If you buy an a/c based on the amount of discount available, you are likely to end up only buying the one that was most overpriced in the 1st place.
