My current 6mm interests: orks, titans (chaos, ork and amtl).
I've also got future plans for tyranids, necrons, tau, squats, IG, eldar and dark eldar (basically all
)options:
1) Dark eldar raider - looted for my 40k orks (display army only now) with bitz for the chaos titans.
2) IG sentinal - legs stysem used as basis for titan
3) 5 possessed chaos marines - easy enough to modfiy into chaos knights, + chaos bits
4) Chaos termis - have to chuck an extra 8 quid in, but would work as basis for heavier duty knights / warhound
5) as above, but imperial marines for imperial knights
6) space marine dread (no chaos one available, 8 quid over budget) - used as basis for warlord titan.