I'm no expert but I would get rid of all of the old paint. There could be some rust under some of that paint. I normally use 80 grit on the bare steel with a DA then a few coats of epoxy primer to protect the body. Then I block sand the primer with 220 then 320 then paint. Some people go as far as 1000-1500 on this but I never have it it's always looked great. But I don't do 100 points trailer queens either. If you choose not to strip you can use some sealer to prevent any reaction between the differnet coats. You can always try a small section to see the reactions also if not sure.