i agree with rehab. if you don't want to do any modifications at all, the later clubman seats a good. i recently put a high back seat from a Leyland Mini S in my 850 that i use around home. i personally think it's a good improvement, but the seats that were in it were pretty poor! The high back seats are also alot safer for your neck in case of an accident, however it may make rear seat access a little more difficult!
Unforunately alot of the topics regarding replacement seats have disappeared from mini chat, so hopefully some people will be able to help regarding mods needed. i'm putting mazda seats in my current 850 resto project, but it's with a few permanent mods so not really of any use to you
evan.