Airbabylon,
First of all once you know what % share you are going to own i.e. 20% (or a 1/5 to those who deal in fractional ownership heehee
), have a look and see if it makes economic sense. I.e. If a 20% share is £10,000 then the aircraft should be would £50,000.
Go online, get the ads out, etc and see what similar aircraft are going for. So if for instance you find similar ones for £40,000 you know it is overpriced, likewise if they are £60,000 you could be onto a bargain!
Check the kit inside, ask about any bills they know are coming up (when I bought my share I knew a respray was in the pipeline), try and make an assessment of the general condition. If in doubt get an engineer to have a look for you.
When you buy your share you should be given an invoice for the amount of your share and stating what % you now own.
Dont also forget that should the group put a fair wedge of cash into the aircraft to upgrade avionics, respray, etc then a revaluation of each group members share may be order.
J.