Re the price, you will get something at every level, and you will get something pretty good for £100k. The main thing I would watch is the avionics, which on an IFR plane you want to be working! The panel alone in a decent IFR plane is worth a lot more than most Cessnas and PA28s you see around.
If you want an IR then (assuming a PPL) N-reg is the only realistic way. There are a number of firms who will set up the US trust for you and run it afterwards. You are then under the FAA regime; the CAA know nothing about the plane. I can't say more because I am working towards this myself and haven't yet sorted it all out. But you need at least the FAA PPL before you go to N-reg because the FAA requires that an N-reg plane is flown with an FAA License (anywhere in the world).
Personally I would get the PPL and do some 30-50hrs at least before going for the IR. But actually if I could re-run things I would have bought my present plane and did the PPL in it; the normal way (a PPL in a C152 or similar) only proves that you can drive a 2CV. Anyone with intelligence and aptitude can easily master the few extra bits in a decent plane to start with.
The ltd co. ownership issue depends on whether there will be other pilots (it should protect the owner from personal liability if someone else crashes it and the insurance doesn't pay out) and whether there will be business travel (easier to make a case if the ownership is in a company). Some owners own the plane in the business which is their main livelihood; this is good if there is a lot of business travel and should be OK if you are the only pilot! There are various angles on this one. But unless you do lots of business travel, or rent it out, you won't be able to get away with full VAT reclaim. A good accountant should offer tips here.
I don't see how magically your club could find you a plane which you could not find yourself. That proposal is bizzare, IMV. Do they think you are stupid but have lots of money?
The socata.org site is pretty international. There are a few used planes listed there but there are many more out and about, and perhaps the best ones are't advertised (yet). If you look for a TB, call Pat Patel at Air Touring and ask him if he knows of anything available. He also knows the history of most UK TBs. You will need to check out any other plane in a similar way - don't rely on the owner saying "nothing at all has gone wrong with it in the last 5 yrs".
I hope this helps. It's just my view and there are lots of other things to consider. But there aren't many planes on the market in that category that aren't too old so that will be the hardest job.