Hi,
3 months is too short in my opinion, even if you work everyday. But is possible depending in your experience, parts and cash availability.
Other important question, if you are doing a real restoration do it as better as possible. Don't forget certain question. Do it properly, you can avoid re-do it again...
About certain mods that you think that you should include, think always in the use that you will do to your Land Rover.
My experience tell me this. I rebuilt a SIIA of 69, mainly to some off-road (light and safe ) and small travels. But it past months without running, because I had no time to enjoy it, and I found the LR with minor faults due to the lack of driving. Then I decided to use it in daily basis, not everyday but at least 2 days per week. I found it not suitable to do this properly. Thanks to that at the moment I'm doing some mods.
LT 77
LT 230 (from a 90 2.5 n/a)
axles from a disco 24 splines (with all the safety items included)
PAS
About the engine I'm limited thanks to Portuguese legislation, but if I live in the UK my choice it would be a 200/300 tdi.
A better engine fit better in spec presented, but at moment I'm certain that I gain in comfort and drive ability.
Good luck