Plasmas are a good time - you'll never want to go back to any other cutting device ever again.
I've got a few around my shop:
One is a very old Thermal Dynamics PakMaster 25 (25A output power, 110V input). It cuts well up to about 3/16", and can make its way through up to 5/16" if you don't mind a more ragged cut. This was my only cutter for a long time, and has served me well.
The other is a new Thermal Dynamics True 39 (http://www.weldfabulous.com/Plasma-C...-p4543781.html
). Great price right now! It happily cuts 5/16" with great kerf, and can be forced through thicker materials at a sacrificed cut quality. It is a 39A output, 240V unit.
The 110V unit we have is portable, which is nice, but would be a little weak for certain structural elements on a truck. The larger unit lacks portability, but is definitely the better unit.
A few pieces of advice - buy a name brand. Lincoln, Miller and Thermal Dynamics are all recommended, but a good unit will last many years, and you'll need to have enough dealers to buy all the consumables, tips, electrodes and cups. Our little Thermal Dynamics is over 20 years old, and I can still walk into any welding shop and buy all the consumables it needs off the shelf. That wouldn't work with an off brand.
Avoid 110V units unless you need the portability. You really want more than 25A output to cut the structural parts of bumpers and frames, and more would be better. Any more than about 50A is probably not going to be useful to you though.
Make sure you have a good air compressor (doesn't have to have a lot of CFMs, but it should be a reliable unit).
Put a GOOD water and oil filter between the compressor and the cutter, preferably mounted as close to the cutter as possible (perhaps right ON the cutter, at the end of the air hose). Water and oil in the lines will make your cuts bad, and destroy your consumables. Note that this is exactly the opposite of what you need for air tools, which is to have an oiler in the line to intentionally keep oil in the lines. If you use air tools, set up a manifold right off the compressor and run a dedicated line to the cutter's filter.
Shop around on pricing - we've bought a few welders from Weldfabulous recently and they've had great prices and cheap shipping (which is a big deal for something this heavy).
Used is fine, but make sure it is a good brand, and check the two easiest places to damage - the ceramic cup on the torch (which is replaceable, but expensive), and look for abrasion or damage on the torch line (easy to step on, but runs both air hose and electrical, so it can be fragile).