I'd go for a grown up
1/2" set and I think you'll also need a torque wrench. If your yet to strip down your donor, your going to need some severe leverage on some fittings so a buster bar might be useful, plus the whopping 42mm socket for the driveshafts. I imagine you'll want something like 8mm - 24mm (all of which I've used on my build so far). Like Andy says, spark plug sockets (long sockets in general) are also really useful.
I've never liked the smaller drive stuff, I've snapped a few smaller sockets trying to undo bolts on cars. Personally I'd get a set from Machine Mart or maybe a brittool set from screwfix.