Hi, the etch primer is only needed on bare Ali. Where there is still solid paint just key it and use normal primer.
Presume you will be using 1K paint (single pack) buy the best you can afford this will make all the difference, warm the paint to make it thinner the less thinners you have to add to get it to flow the better ( max 10-15% thinners) although you need to get the paint thin enough to make sure it atomises or you will get a nice orange peel effect!
I add some 2k fast hardner to the 1k paint I use to make it go off quicker ( max10%)
1k synthetic dries more with airflow over it rather than heat so don't need to heat the makeshift booth to ridiculous temp.
Don't get to hung up on the best spray gun you can afford, better to spend the money on better paint, just make sure the nozzle is the right size for the job. One for around £50 should be fine, I've got a kestrel one with 1.7mm nozzle for 1k and devilbliss with 1.4mm nozzle for 2k
You may to get some 1200 wet and dry in as well incase you have to sand the top coat once dry to spray again!
Good luck and be patient!