Good decision.
As Stuck said, seeing as you dont already have an engine, aim to make the biggest you can, which is 4.6. You can go bigger but it will cost £££££.
RV8's are difficult to tune on a budget so going for the biggest is the cheapest way to more horsepower/torque. EFI is essential for any kind of offroading, and if your budget will stretch Megasquirt is highly recommended. Give Nige (hybrid from hell) on here a shout.
If you need a step by step on building your engine, give me a shout. Happy to help.
Edit: There are always loads of R380's around ebay but bare in mind you will either need one from a defender, or you will have to change shaft in gearbox to suit..