These are questions that are continually debated in autobody circles. I would say the consensus is that any metal that is going to be finish painted needs to be cleaned to bare metal, no rust, then sprayed with epoxy primer. I do believe that most folks are not as concerned about the epoxy primer matching the manufacturer of the top coats. A lot of people use PPG epoxy, but it is expensive. There is a cult following for "SPI" epoxy which generally has to be ordered and shipped. Do a websearch on this. After epoxy, then body/filler work. Spot spray the filler with epoxy. Then a high build 2k primer. Guide coat and block sand until smooth. Seal coat, base coat and clear coat, or alternatively a single stage top coat.
I have always heard that it is advisable to go with one company's system due to potential compatibility issues, but a lot of folks do not follow this rule, including me.
Every paint guy that is out there can tell you the best paint there is . . . . . its the one they use. I believe any of the major brands will treat you fine. 2k Urethane is the standard these days, enamel is yesterday's technology. I am doing my first stab at painting with my '50. At this point I am using Valspar products as reasonably priced good primer products and Dupont paint.
Most folks will tell you that POR is not designed to be used as a primer. It takes a long time to fully cure and should not be top coated for some time. Generally POR is used on frames and in locations that are not highly visible.
If you are just working at a slow pace right now it is your best bet to just get everything to bare metal and epoxy prime. This will keep the metal rust free while you work on your truck. Most epoxies have a recoat window of about 7 days, but you can go years without top coating as long as you scuff, clean and shoot another coat of epoxy before topcoating.
I can tell you from experience that there is a mountain of information and opinions out there on paint. It can seem overwhelming but you can figure it out. Try to find a auto paint vender that you can work with and that will take the time to explain things to you. However, do always take their advice with a grain of salt until you can figure out if they really know what they are talking about and can be trusted.
Finally, think safety, the modern paints are nasty stuff. You will need good ventilation and at a minimum a good, proper fitting mask made for paints. A lot of folks would say the $400 or $500 hobby style fresh air system is the minimum you should be doing to protect your lungs. I know that seems like a lot of money, but I hear lungs are worth a tad more.
Good luck
