You might also consider looking at commercial units?, then price up what the parts would cost you! - it's probably not cost effective to build them yourself?.
It's so obvious I imagine you already know?, but a model car speed controller is probably identical to what you need.
well i don know how big a heat sink u ll need with those mosfets and may be some additional cooloing arrangements..
check out irfz44n, its gud...
why dont u go for relays here... i think they ll be better.
They don't normally have any heatsink at all (due to size constraints), they commonly use multiple parallel MOSFET's, with the tabs sawn off to reduce their size even further. The multiple (low resistance) FET's mean there's very little heat to dissipate - essentially the more expensive ones simply stack more FET's for higher power handling, and less voltage loss to the motor.
why dont u go for relays here... i think they ll be better.