We're always nice to newbies!
There have been quite a few 4.5m secondhand RIBs for sale at very modest prices... Wavelength had a very nice Avon for sale recently complete with Yamaha 50 o/b and trailer, about £2k if I remember correctly. Also keep an eye on Ebay, boatsandoutboards.co.uk and the "Buy a Boat" magazine for bargains. A professionally built boat, fully rigged, might look attractive against self-build. And it will probably hold a better resale value.
I'm no boatbuilder, but I think that plywood in RIBs is usually used just for stiffening, with different types of glass-fibre sheet added at appropriate places (the lay-up) and a gel-coat finish. I understand that these materials can be quite expensive and it's a skilled job. Also, many boats boast of a "wood free" construction.
Depends what you want - is the objective to enjoy building your own boat, or to get out on the water in a fast, seaworthy craft? (not that the two objectives are mutually exclusive!)