Well I am going to disagree a bit on a few points....
Go big. I have a Thermaldynamics 102. The salesman didn't want to sell me the 100amp model, but I am glad I spent the extra $600 on it. It cost just over $3000 at the time, and came with a promo of a small portable 110V welder. The plasma spends most of its time with 80amp and 60amp tips in it, with the 100's and 40's getting used less often, but still get used. There is rarely a day that is doesn't get put to work on a job of some kind. The torch still gets used, but only for heating stuff and cutting off bearings etc. There is very little slag with the plasma, and what is there is easily knocked off. Only drawback of the 102, is that it needs to be direct wired, as plugs that large are cost prohibitive, and likely to fail. Mine is on a boom that can telescope out into the shop. If you need more length, you can get lead extensions. The other thing to make sure of is to keep an extra air dessicant/filter on hand. It will work extremely poor with damp/dirty air.
I look at the size as of the plasma the same buying a different tractor. Would you buy one that is just big enough for the equipment you have? Or would you like a few extra ponies for those few times you need it?