Welcome to Pprune. I am sure you will get lots of answers but, to get the ball rolling, here's mine.
Buying anything involves some kind of risk. The main risks in owning a share of any aircraft are:
That the owners do not work together to maximise serviceability and minimise costs.
That if something goes wrong you have to pay your share to fix it. Most groups build up a fund for major expenditures and, when you join such a group, your payment should cover your part of that. If the cost of repair exceeds the value of the fund, then all members will have to put their hands in their pockets.
In my view, buying a 5th share only makes economic sense if you do at least 2 hours a month - but when does our hobby ever make economic sense. It is worth it to be able to use the aircraft for longer trips than usually possible when hiring.
For a 5th share costing £5 -7K I would expect a pretty good PA28 or equivalent or an older Permit aircraft with a good engine fund.
When you leave, you should get your money back when the share is sold. This may take some time.