Do I really need a flyback diode when using a power mosfet?
If you are using a full H-bridge (or half-bridge) the body diode of the other MOSFET (the one not switching) will act as a flyback diode, however the power loss may be more than through the MOSFET since the motor run current multiplied by Rds(on) can be less than a diode drop for a good MOSFET. 

The Vds will thus be clamped to the battery voltage (plus a volt or two), but take care that there is not too much inductance between the H-bridge and the batteries- otherwise the MOSFETs could see too high a voltage and avalanche. A sturdy TVS right at the bridge (across the power rails) can be a good investment (definition of sturdy depends on how much current and how much inductance).  

Obviously you cannot put a diode directly across the motor (or it would burn up as soon as the motor is reversed) but you can parallel the MOSFETs with power Schottky diodes. That will reduce the dissipation in the MOSFETs, depending on how you are driving the H-bridge. 

As jippie and others have said, the package (wire bonds and such like) limits the drain current to 75A and that's not very conservative. You actually may have troubles with such a modest MOSFET running a wheelchair motor. In particular it will tend to fail when first started because the battery and motor resistances are very low so the peak current can be very high. 

I hope if you're making a wheelchair type device you have a manual override to disconnect the battery. MOSFETs (like other semis) tend to fail 'on' and I can visualize some unfortunate possibilities.