How much is a C of A ?
Your best bet would be to speak to the engineer who maintains the a/c and ask him to give you an honest appraisal of anything that is up and coming on the a/c. He knows you will be the one asking him questions if he hides something from you and you will get an honest answer from him.
I have never been a part of a group and have been fortunate to be able to be a sole owner. The one thing that would concern me about a group is the integrity when it comes to major maintenance and an obvious "desperateness" to get you to buy in. Therefore it is up to you to do your homework.
Ideally you want to get involved in a group with an a/c that has low(ish) engine hours, functioning avionics fit and a good safe history.
Personally I think the 152 is a brilliant a/c. It's not perfect at anything but can do everything quite well. Too many a/c (like cars I suppose) excel in one department but disappoint in another. By buying a share in a 152 you will be limiting your risk somewhat due to their robustness.
I would say a "normal" C of A will be in the £1300 to £1800 bracket. It should not normally take more than two weeks but this depends very much on how many a/c are queued up when you drop yours off!
Good luck.....