1. all conversions require rooting around. Otherwise it wouldn't be called a conversion. Some just require a lot more than others.
2. Any conversion that will put out more power will require a brake upgrade. Otherwise it might be a break downgrade. and with all these bent celica's around of late, I think it is a dam good idea too.
3. By the sounds of things, you want to go efi. My opinion and it might be biased is to go for a 1G-GTE. Heaps of power, great engine to drive and it isn't all that hard'a conversion. Check out some of the tech articles here for more details.