1 - Dry air: I never ran mine without a filter/dryer so I can't comment like Dan did. A good one is not cheap. I got the Motorguard 28 kit (it is recommended that air be clean AND dry). The last time I looked at the filter it appeared to be ok so I have not changed the original one.
Here's a link. I think I got mine from Air Liquide, This link is from Amazon (model 30 is a bit larger than my 28 I believe)
2 - Compressor size: I have two compressors: a CT Maxium 15 US gallon, 4.0 cfm @ max 150psi; and a King Canada 4.6 gallon, 2.3 cfm @ max 120psi (KC Flex-Air model bought from KMS, free shipping). They are both sold as "Ultra Quiet" - the larger one is fairly quiet but the little KC lives up to that claim (& is really quiet). Ahead of my hose reel is a little headed Y so I can run both together in parallel. I mostly just run the little compressor. I team them up to operate the die grinding, air nibbler, or plasma cutter. Higher maximum compressor operating pressure substantially increases the effective tank volume. Plasma cutters normally operate at 70psi.
3 - Cut time: I cut my RT fixture plate from a piece of 5/8" plate. As I recall the max cut length was about 15" and the plasma cutter did not run out of air. FYI - In the picture above (post #78), I used a scrap piece from that project for my example set-up. My 45 amp machine cuts 3/4" plate OK but not as cleanly and smoothly as 5/8". I did some 1" plate just to see if it would. It struggled, thus IMO, at that thickness oxy/ac matches the cut quality of a plasma cutter (depending on your experience/ability/size of plasma cutter).
4 - Googles: UV protection is important. I've seen either shade 3 or 5 being recommended. Most welding hoods don't go that low. I use my Optrel hood, it goes to shade 5.
5 - Gouging: Not all plasma cutters have consumables and machine settings designed for gouging. This is one of the benefits of springing for a premium-quality plasma cutter. IMO the Hypertherm's gouging functionality is awesome (controllability, smoothness, flexibility, accuracy, etc). Gouging with a plasma cutter is a joy! I do not know if other brand-name plasma cutters can match the Hypertherm when gouging, but I expect they surely can.
PS 1 - I have no plasma cutting training. I'm self-taught, so it would not surprise me to discover that some of my comments are incorrect.
PS 2 - WOW - while tapping out this post - I have been listening to the radio. It has been hard to concentrate and it took a lot of time to finish. What a day in Ottawa!!!!!!!!!!!!!