If your after that 'bufty' look, i would use an etch primer, then a 2 pack top coat, it's very tough, is good at holding back any rust in seams etc, and looks the part. If you want durability, waxoyl over the top of the 2 pack, it'll look sore, you'll get covered in it when you work on the underside of the car, but it's the price you pay if you want to protect you metal work from tin worm.
To clean up the underside, i first de-grease the area to be worked on. Then i used 'web wheels', you use them in a drill, they look like disks made from like a scouring pad. They're excellent at removing seam sealer, paint and light rust. They aren't the easiest things to get hold of, 'Frost' do them, but i get mine from 'RS', but for them you will need an account. I'll post up some further details when i get back to the workshop. Black and Decker powerfiles are top too .
Powder coating is supposed to be excellent, as long as the parts being coated are free of rust. The only horror story i have heard was where the coating lifted on a part, moisture got in behind it, rusted the component away to excess, with hardly any visual indication from the outside. Though i wouldn't let that put me off. I was also told of a rally car, that all of it's rear suspension components coated. After the years rallying, they jetwashed it all down, and it all looked like new!
If you can sort out a jig to turn the car over, it is the way to go, not only will it be easier to clean the underside up ( no sh*t raining down on you ). It will also be easier, hence better job, for the person painting it!