Ive never drove a Mk 5/6 RS2000 so cant coment on them.

But ive driven a mk3 RS1800 and own a mk3.5 RS1800 and i rate the mk3.5 a lot.
Not the fastest car out but an all round fun car.
Not the easiest or cheapest cars to tune.

Throttle body conversion costs around £2000 and gives roughly 30bhp.

They are reliable cars though.