Do you mean for only hour building. I suspect you may mean for CPL/IR training.
For hour building it can be a good idea. You must be organised though. The only problem is if anything goes on the aircraft you may be hit with a big bill. Some schools have frequent flyer schemes where you pay monthly and then an hourly rate. Have seen some at around £60 for 152.
If you are getting into a share I would recommend one with at least 10 members. Costs are then minimised, and most large groups I see never have an availability problem. Also if you have trouble selling the share later, you don't have too much cash tied up.
I'm beginning to think BL is on commission from the SA tourist board!!
She raises a good point. You can go to the states or SA and get 152s for around £35 an hour. Buzz is trying to flog a share, don't listen to him!!
If however it is for training in I think you will be getting in it up to your neck. You would need a multi if it's for an IR, or at least a complex single. Lets assume a multi.
Initial outlay would be at least £50-60k. And that won't be for anything decent. 16 gph burn - rubbish for hour building in. Horendous maintenance costs - two engines now! Avionics need to be decent - and mode S comes into force soon to add to your worries. Then there are any unforseen problems. As an example Tayflite bent their Arrow back In aug/Sept. It's still not back (although this may have changed recently?)
Then you need to find similiar people brave enough to take the same on.
Also many schools are so busy they wouldn't entertain the thought of instructing in an a/c not in their fleet. Why would they if they make more on their own. If they did they would charge a lot for the instruction to cover the lost revenue from the a/c.
This is all my opinion. I hope it helps.
All the best
SK