The crossmember and the loom are the only real issues.
For instance, the fusebox took some work to mount and is another area I'm not 100% with..
Also, the later cars have two big holes under the battery for different bits of the loom to come through, but my 4i only had one - I managed to get all the wires through that though.
Then fixings for ABS etc had to be fabricated (3dr may already have these).