......... Dry air is important but having an air compressor to keep up with it is important to....
ABSOLUTELY!
I purchased a Hypertherm 1000 in 2005. It is one badass cutting machine! I get my consumables from E-Bay for about 1/2 retail. It's cut 1/2" plate at about 30" per min.
I burned up a Craftsman compressor right away chopping up a 1970 Ranchero. But I was running it 100% duty for 1/2 hour. I went with a USA Curtis - Toledo and haven't looked back.
I use two in line filters and a dryer from the compressor. Moisture will kill the machine quickly, and irrepairably.
A Hypertherm Powermax 600 should be all you need for automotive work.
I disagree with getting a 3 in 1 machine. If one part goes down it may affect the whole unit and may not be worth repairing.
A plasma cutter is not best for all cutting thou. Tremendous heat, spraying molten sparks, and slag make it unsuitable for a lot of body stuff on a put-together-auto with fuel lines, flammable materials. I use a good quality Ingersoll Rand Pnuematic Body saw which does well on body work without the sparks.