Well The500man I would under no circumstances borrow the money for a share - that simply adds too much risk - what happens if a large repair bill lands? We threw people out of the group for not paying their share and given that flying is pretty tight knit sort of hobby it is a bit like being given the 'black spot'.
Given your concerns of depreciation of the value of the share that either means there is a lot of money involved, or money is pretty tight - neither is a good idea to borrow for.
As for getting your own engineer to check out the group wned aircraft - not a good way to introduce yourself to a social grouping and certainly in the two groups I was involved with, we would have told you to 'sling your hook', probably in a rather less polite way! And it will cost at a minimum around £300 - it is money well spent?
If you are that worried about the value of your 'share' might I suggest a 'non-equity' group, a little cheaper than hiring through a club (or it should be otherwise there is no point!) but probably more expensive hour per hour than a share - if you compare like for like.