Hi,
1.- N channel MOS are more common and less expensive, and not difficult to drive with 24V DC rail.
2.- You HAVE to regulate current by switching. Otherwise you end having some some sort of audio amplifier instead a motor control.
There are many ways to sense current. You could use SENSEFets, or try something like a IR2175
http://www.irf.com/product-info/datasheets/data/ir2175.pdf
Keep in mind that if you design your own current sense circuit using a resistor in series with the motor winding you'll have a huge common mode noise.
Best regards