I am very much an amateur when it comes to painting cars, I did paint some cars when young with lacquer, but all that stuff is obsolete now. Currently doing a 1971 Mach 1 Mustang. I bought the Harbor Freight drum sander, which is a copy of the Eastwood. It does a great job at removing paint and leaving a really nice surface to work on, and it will not heat warp your panels (I guess you could heat warp them but it would really be a user error). Anyways, taking a car down to metal and starting ALL the body work again is a monumental amount of work. If you have good solid paint on your panels, nothing that is cracking, or spider webbing, it is a good idea to use that old paint as a base. Most body shops will not take a car down to metal unless the customer requests it, and pays the huge fee it will entail. If you are not a professional, and you have a body panel that looks good to great with good old paint on them (that is not lacquer), I suggest that you work over what is there. When you start going all the way to metal, what you find underneath it may be a nightmare, and doing all that body work again, especially when you are not a professional is a daunting task. Ask me how I learned this... Regrettably I have some very old paint, that is cracking, and has quite a bit of issues in some parts of the car. Some parts of the car will go to bare metal and I will try to work with other parts of the car that still have good paint. I know people will argue that the correct way to do a car is to go all the way down to metal, but they are not the ones doing it. At the end of the job, if the car looks straight and the paint looks good, who cares if you went down to metal or not? The argument will still go that going down to metal will make for a really long lasting repair, but if what you have there has lasted to 10-20 years or more and was done by a pro, you can argue that what you do as an amateur may not last as long, and may not end up looking as good. If the body work lasted 10 years, it will probably last another 10-20 more...
As for the painting, doing a car pieces at a time is usually not a good idea. You really should paint the whole car in one shot. When doing a car in pieces, it is common to have issues with the color not looking the same on every panel, if you are using a metallic paint, forget about even trying to do it in pieces. I wanted to paint my car with a single stage paint, but after much reading and asking questions, I decided to do a base coat clear coat. Why? Because when you start painting with a single stage paint you basically cannot stop to correct issues that occur during the painting process, once you start you need to finish. I do not have a paint booth and I would be willing to bet you do not have one either, so stuff falling on the paint, runs, or anything else that pops up on the paint, when doing a single stage, you pretty much have to live with. On a base coat/clear coat you can stop in between coats and sand out/fix issues that pop up. You can let the base coat dry, sand off the imperfections, respray any base if you have to and then go to your clear. When doing your clear you can stop before your last coat of clear, again fix any imperfections, sand it down if you have too much orange peel, and then apply your last coat. Base coat/clear coat is much more forgiving for the amateur. At least that is the way that I understand it from the research I have done.
Tons of paint brands out there, I have a PPG dealer near me. they have the Omni, the Omni Plus, and then the high buck PPG Deltron stuff. I have heard that the Omni stuff is a cheap paint, and not very good, and that it does not cover very well, so you have to do more coats to get coverage. It seems that the Omni Plus is the sweet spot, and the PPG Deltron stuff is top of the line, great to work with but stupid expensive. There are now a bunch of online companies selling paint online, some seem to be really good quality, with very affordable pricing, so do your homework.
Use the urethane under the hood, in the jambs and in the trunk. You flew helicopters....so you can paint a car! 
