Hi guys....have rear axle, trans removed from my car. I have all winter now to putz on it. Goal is durable, painted underbody (for now, just the rear half of the car).
Car is southern car with minimal surface rust (more dirty than anything). Here are 4 preliminary ideas I have as of now. I'd rather not sandblast and deal with grit everywhere.

Do I:
wire brush it and wipe it clean - Rust Bullet, then topcoat body color?
wire brush, acid wash, rinse with water (not sure i feel like doing this under the car) - Por15, then topcoat body color?
wire brush, wipe clean - just epoxy primer, then topcoat body color?
Wire brush, wipe clean - Eastwood rust inhibitor, Eastwood rust encapsulator, then topcoat body color?

Garage is unheated, so chances are that I would have to wait until next spring to paint, if there's a product that can be applied in cold, great.
What would be some of your suggestions? Thanks!!

It could be done in 1 hour if your only wanting to to do the rear, maybe 2-3 hours if you wanted to do the whole bottem.
Rent a heated pressure washer, wear the appropriate clothing and face protection and get under there and do it. Rent one and be done with cleaning it in one afternoon and spend the rest of your winter doing something more important, work smart. The wire brush idea is for fools, sorry.

Yes, use a hot water pressure washer that makes STEAM. 300 degrees +. That will soften all the undercoating, and melt and blast away all the grease and goo.

After you clean it you will still have to deal with the surface rust situation. Then spray with epoxy. If you leave it in gray epoxy it will be closer to original. They were never painted body color on the bottom.

so once it's cleaned, what's the most cost/time effective way to deal with the surface rust?? can I paint the epoxy primer over that?

For a driver type resto, I roll on some semi flat black POR-15. Zero Rust may be better because you can get a closer color to the dip gray.

Its a messy job no matter what, but I generally use a wire cup brush on a 4-1/2" grinder followed up with mineral spirits to clean the underbelly.

Go to your hardware store and get a few paint trays for the 2" or so rollers. If you lye on your back and relax, you can roll most of in on quickly. Then fill in the gaps with a brush.

Ideally if you want to do it once and do it right you should sandblast. Maybe you can use some tarps around the bottom of your car to control the dust from going everywhere in your garage.