This really has been done to death in thread after thread.
The reality must be that if you have to ask the question you probably can't really afford it!
As an outright buy though the cheapest aircraft will be one old enough to be interesting yet viable as a regular mount, but not young enough to be on a C of A. So, things like early Cubs, Aeroncas, Luscombes and Jodels will be cheap to buy - £12,000 say for a Jodel, perhaps £22,000 for a Cub - on a Permit to Fly and some will run on mogas (all will run on 91UL when it arrives). A rotax or Jaburu will probably be a lot more expensive to buy - £60k+ for a Pioneer, Sportcruiser, MCR, Tecnam etc, but may be a little cheaper to run than the older permit stuff, certainly in terms of litres per hour and of course they will knock a Jodel into a cocked hat when it comes to cruise speed (the Sportcruiser is a bit of a poor performer in that department though). At that price however you have to allow for a fair chunk of depreciation - the benefit of an old Cub or Jodel is that there is little depreciation if it is correctly maintained.
The most expensive option is without a doubt a new C of A type followed by an old dog of a C of a type. Let £8k maintenance per year for a C172 be a warning to you. If you really want to fly one of those get into a decent group. In fact, unless you are flying over say 75 hours a year a group makes a huge amount of sense, as even a 1940s vintage C65 engine is going to be costly if it goes wrong and being able to share that cost is hugely reassuring.