There are various rivers you could explore, and beaches you could launch from, but I reckon the best SIB cruising area in those counties is Chichester Harbour.
Definitely pneumatic wheels.
There's no definitive answer to your third question, as it really depends on you. In calm conditions you have three limiting factors: how much fuel, what you'll do if you break down, and how long the conditions are likely to stay good. I'd suggest getting a bit of time on the boat close to shore before you get too ambitious, and an RYA level 2 course would give you a good start to be able to decide the answer to this sort of question for yourself.
Have fun and let us know how you get on