Welcome to Ribnet - sounds like you have a fun project on your hands
Have answered what I can ......
1. Do you think i should chop it up and stick in bin? (its solid all over, its down to bare shell with only few nics) - Noooo I think it sounds a great idea and could be real fun
2. You were going to sand hull down with belt sander to lightly to take wax off gelcoat and gelcoat hull a different colour - do you need a primer before gelcoat and can you put 2 layers on with colour and 1 clear? - No idea sorry
3. You need advice on everything from electrrics to retubing? - lots of advice on here do a forum search and you will be amazed at what comes up!!
4. Is retubing expensive? - from what I can gather and depending on the size of your RIB about 2k for a 5m RIB so I assume less for a 4m but not an area I have any experience of
I'm sure you will get lots of helpful advice - personally I think it sounds really exciting