Good luck with whatever you choose stevo_sideways. There's a few choices that others have already done. It is hard to tell - from the different engines, approaches to installation, tools and skill involved and required - what the best choice would be. I suppose the 'what you fancy doing' factor is a good guide to making the choice. This is what others have done and finally felt vindicated by in the end.
I know you said you'd prefer to keep it naturally aspirated... but... wouldn't have considered a turbo myself until I drove a few of Ant's turbo conversions. I'd have one. M20 turbo seems so much beefier than other turbo cars - which are often 4 pots. It drives like a big engine. And has lots of torque.
If a turbo is definitely not on the list then the E36 M3 engine would be a cool choice. A few others have obviously done it but it looks tricky and can takes ages to source all the right bits and do all the little mods required.
Another obvious choice 328i E36 M52 2.8 engine. Easier to get hold of, cheaper than the M3 and would deliver the bhp you mention especially with mods.
Considered keeping the existing engine? I love the M20 engine and, although the high top BHP figures potential of engine swaps are attractive, it is rarely possible to use much of that peak power on the road (although you do have a lot of open road over there in Ireland which would be silly of me to ignore). A high torque set up is often more useable. This would be great for the Green Hills of Ireland. I'd go with Ant's suggestion of keeping the. M20 and 2.7/2.8 conversion, hot cam,TB's Aftermarket ECU and also maybe a six branch manifold. This engine set up delivers loads of torque and would be a nippy little beast.
Looking forward to photos of what you do over the next few weeks then
