Damian,
Some help with point 4 (sorry no idea about the other 3):-
- do a search on allatp and Riverside and see if anything relevant has been said
- do you want to fly in the US once you qualify or come back to the UK? If UK then you will need a JAR licence which will mean the conversion of your foreign qualifications. Amongst other things the sitting of 14 bl**dy hard exams! Again search on conversion and you will get hundreds of threads about the ins and outs
- I have no idea about the FAR's but think that you might be able to find out something on the FAA website (or buy one from Transair or similar - FARAIM is its name) to see the legalities of having an instructor teach you in your own plane. In the UK there are rules covering the remuneration of instructors and they differ depending on whether the aircraft is public or private category
- search on the private flying forum for information on purchasing aircraft - lots and lots of pitfalls, lots of benefits
- I am not sure buying an aircraft for your training is the right answer unless you could syndicate it and maybe cover the costs from other people. Reasons are that you will likely train for your PPL in a light single, do some hour building in the same, go onto your commercial on a more complex aircraft (eg: Arrow) and then multi (obviously on a twin engine) and maybe more multi hour building. 3 different types of aircraft alone.
Good luck.