What do you mean by 'transient voltage suppressors'? Is it similar to a snubber (RC to GND)? That is something that I'm considering adding.
I mean a transient voltage suppressor diode - google "TVS diode" for info. They're like Zener diodes, but instead of being designed to have a very flat voltage characteristic in the reverse direction, they're made to break down very quickly and to be able to handle very large surge currents without damage. They're often used in parallel with devices that have a limited voltage rating and which might be damaged by inductive spikes and other noise - like a MOSFET, for example.
Do you have any advice for component power ratings?
How long is a piece of string? Work out the actual power you'd expect to be dissipated in each component, given the load it's driving, then add whatever you believe to be a sensible margin. Consider worst case fault currents rather than normal operating conditions too, you don't want something that works fine until the motor stalls, then catches fire.
IMHO if you're making a small quantity of something, then big, higher rated components are the way to go. The extra cost is minimal compared to the hassle of a repair or redesign.
With regard to keeping the current circulating, I was going to activate the low-side FET of the hi coil when PWM is low (if that makes sense). Are the body diodes not enough? And with regard to the FETs, I'm using all N-channel, so hopefully that clears up your confusion.
I guess they might be OK - just wanted to flag it up as something to check. Don't rely on being able to turn a FET on instantly, it takes time to charge the gate.