I would use etch primer on any bare metal.
Underbody, a squirt of "stonechip" paint over the etch primer (NOT underseal) as it resists chipping nicely. Topcoats over, or chassis black. I used chassis black, all ready for future repairs!
On top. After etch, use a high build primer. In fact, if you are paying a professional, better to get them to do the primer coats. That is where the final finish is achieved. Or do it all yourself. (it will save a fortune. Plus the topcoats are the easy part)
Not sure if the Non-isocyanate 2K paints are good? if they are, they may be the easiest to use?? Cellulose is hard work!