I am not so sure about the best suited series but if i am going to make a mod for my car i would use a B series . I see most of the people are using B series and for the transplant i guess if you have RM900 it can be done no prob even double upgrade. I got a friend's ori 1.5 upgrade to 1.6 now he want to upgrade to 1.8 and he said there is jalan . so i guess it's not a prob for 2.0 .
hope this is helpful...:P