I am doing the same swap right now and there are many items to consider but as far as "must have" you will need to retain the passenger side 8v engine mount for the 16v. As previously stated you will need to replace the exhaust system . Others more knowledgeable than I can provide you with other "must haves". Personally I plan to do the brake conversion. I am also using the complete 16v harness to include the fuse box, not only because it's better, but also because I don't want to do any looming of the MK1 wiring harness.
David G
1981 "S"
1980 (wide-body 16v to be)