Before you paint it you must get rid of the rust. The rust will just break right through the paint. Some coarse steel wool might work well to remove the rust. After you remove the rust try looking for a rust prevention product from Eastwood's based on your needs. After that, prime then paint. Do not just paint over rust.
If you clean (to get grease/dirt off) it than primer/paint it, it will say ok. the rust will come back in a few years depending on the conditions it is in, and the paint will easily scratch off. to paint it right, you need to get rid of the rust... i doubt steel wool would be sufficient. it needs to be shiny clean in order for the paint to stick right... i would recommend using a wire wheel on your angle grinder. don't waste your time using one on a drill.
the other option is to use por15. or if you are rebulding the unit, have it sandblasted while it is disassembled. don't do this if the trans is still together, because you will inevitably get sand inside the transmission no matter how well you seal it up.
If it’s not scaling and flaking you can paint POR-15 right on it. If it’s flaking then just hand wire brush it and Paint the POR-15 on it. You must get all of the grease and oil off though because the POR-15 will peal off if you don’t.