I have the Tillett seats in my car - they fit perfectly with lots of room, but as mentioned not a cheap option, though the GRP version is cheaper than the full carbon option.
As well as the seats you need to budget for side mounts, runners, frames/brackets plus a ton of odds and ends - bolts, spacers, etc.. And probably a good day of mucking about getting the position just so.
If you want harnesses as well then you need to carefully plan how you will mount them. However, the Tillets have very low sides so they will work fine with the inertia belts.