I've started getting into auto painting as well. I painted the bumper fillers (plastic pieces) on one of my cars and they turned out ok, not perfect but the car is not perfect and it's WAY better than it was. If you are pretty handy with a spray can you can paint with a gun once you get it set up correctly.
Here's my advice after doing lots of research and actually spraying paint.
1. Safety is key. You will need a paintsuit, gloves, respirator, and goggles. You can get most of that at Lowe's/Amazon or you can go to an automotive paint supply store and grab it. Probably $50 or so there.
2. Prep is the key to getting a good finish. If you half-ass the prep, it will show. If you are painting the old bumper, you need to make sure to get rid of every imperfection. Before you lay down anything, everything needs to be clean.
3. Its a TPO plastic bumper, so you will not get good adhesion unless you use an adhesion promoter. Some guys use Bulldog in a spray can. You can pick some of that up at Oreillys or a paint shop. I am a huge fan of SPI products (Southern Polyurethanes) so I used their Ad-pro. It was maybe a buck or 2 more than the Bulldog $21 vs $18. This is actually designed to spray with your gun.
4. Products - I would use a base/clear system instead of a single stage because its more user friendly. I used specialty waterborne wax and grease remover from SPI to clean the plastic (wont eat the plastic). Sprayed one coat of adhesive promoter. 3 coats of epoxy primer. 3 coats of base color. 3 coats of clear. I did have some orange peel.
5. Follow the tech sheets on everything. It will tell you what temps to spray at, which gun tip to use, what pressure you need to spray at, how long of flash times you need between coats, etc.
6. Keeping your gun clean is HUGE. Tear the gun down and clean it really good before you use it for the first time. There are often oil residues and sometimes (cheaper guns) metal shavings from the machining process.
7. Make sure you're compressor can keep up with your gun. Its really hard to paint anything using a small compressor.
8. There are tons of other things that would help the outcome, but you can probably get by without those (metal cooling lines from compressor, air filters, air dryers, etc)
I got most of my info from autobody101.com forums. I buy most of my supplies from amazon, but some things I've gotten from autobody tool mart online. I also grab stuff at the local paint shops when I need to.