Hmm, presumably you are the same Britemp who posts on mobo chat? If so then I suspect genuine post not a trolling attempt so will respond in kind....
1) RIBs vs hardboats. This could take a year to discuss and would get bogged down in strong opinions either way. But in basic terms - increased buoyancy, softer ride, better in a rough sea, increased safety, SBS machismo overtones etc. Thing is that there are good RIBs and bad RIBs like there are good hardboats and bad hardboats and a lot of it is around how you use it.
Downsides for RIBS are little weather protection, no overnight accomodation (on most RIBs), expensive compared to similarly priced mass produced sportsboats.
Lots of RIBS in the Solent so they must offer some advantages if only as the latest must have fashion accessory
2) 6-7m good size. Short sharp chop in the solent so longer hull bridges it better and makes for comfortable ride. For your budget you are really looking at 2nd hand. So best thing is to look at what you could get for 20k and ask for specfic experience of RIB types here.
3) Yes but will need antifoul on the hull. Other issue is that many RIB's have tubes in the water at rest. So growth on tubes can be a problem. There are solutions to this if you search through the forum or ask.
4) Ideal nav kit is VHF, Depth and GPS. GPS should pref. be a plotter as charts are difficult to use on any small fast boat. With nav equipment the sky is the limit, it all comes down to budget. Being able to use it effectively comes is also important, which brings us to..
5) Training is highly recommended. Doesnt matter whether you get a hardboat or a RIB. PB2 is minimum. A number of good PB trainers available in the solent. I am sure others can post personal recommendations but purely from the quality of their posts here and on mobochat - Tim Griffin, Solitaire/Solent Ranger and Paul Glatzel would seem like good options.
Hope this all helps.