Congrats on your purchase Chris!
So you're going for IVA! Fair play, it's not easy I'll admit, but the satisfaction of knowing it's all pukka and done properly makes up for the sweating and swearing!
Right, my first piece of advice is - gather as much paperwork as you can about your buggy. Receipts, invoices or V5s with your chassis number/engine number on will help the registration process with DVLA. I also kept receipts for all the major parts I purchased for the project. I think basically you need to be able to prove to DVLA that the buggy and all it's parts are legally yours.
Regarding the IVA, the main issues for me were fuel tank and sharp edges. Get yourself a copy of the manual, a cup of tea (possibly something stronger ;D ) and get stuck in! Helps to make friends with your local testing station too, in my experience they're happy to chat and advise.
Good luck with your build, and hopefully we can help you get another one on the road!