Why don't you just rent the aero steed of your choice. Pitts, extras, tigers a CAP 10 and no doubt some others are available.
Generally flying clubs don't make that much on aircraft rental and they generally don't do that bad a job on looking after them. But their hourly figure is about on the number (and they have instructors available)
However, some planes are not available to hire (eg YAKs) so buying is the only option.
If you buy a share find out
A) The state of the finances (are they in debt and trying to recover debt form future rentals)
B) What state is the engine in. All engines have TBOs. If yours is at 90% a £20,000 bill may be on its way.
c) Talk to the owners who will be left behind, and ask around at the airfield. You may benefit from stories you hear at the bar.
D) Look at the ADs. Make sure there aren't any wing spar mods looming on your chosen steed.(£10,000ish)
Having said all of that most groups are lovingly formed around looked after aircraft. The groups tend to expell less than careful pilots. They keep a sensible grip on finances, adjust the figures as required to meet forecast bills.
As to selling the share generally its down to you. Lots of sales of shares are distress sales so cheeky offers are often entertained. If two are for sale then a dutch auction can ensue. Don't feel guilty about this because when you come to sell you will get some ridiculous offers. Don't spend what you can't afford to loose.
Go to a share oriented field (White waltham, Popham, Barton, Headcorn, North Weald ) where you will find an excellent choice .
Whatever you choose have fun.