I've got an old style 75lb Bull, I've used various other pheonix hammers, one 155 Big Blu, and a buddy I forge with a lot has the 75lb Iron Kiss that was John Larson's personal hammer. I've also used various self contained hammers up to 200lb tups, including many hours on an 88lb Striker, and some home built hammers.
If you don't have the air, then a self contained may be ideal, but you can easily run a 75lb Bull/Pheonix or Ironkiss on a dual stage 5hp compressor with an 80gal tank, Assuming that its a higher cfm industrial quality unit, like an Ingersoll, Quincy, Champion, etc. You need a minimum of about 15cfm. I ran my Bull for a long time on a single Ingersoll T30, dual stage 5hp unit. Even running at 125psi, I never ran out of air, even breaking down Don Hanson's big W2 rounds, but that was allowing some recharge time while reheating the work. If I was cold texturing, I probably would have had some downtime.
Now I'm running two IR T30's in tandem, and the second compressor rarely kicks in, and that's only really sessions where I'm pulling one piece out of the fire, as I'm putting the last one in. My friend's Ironkiss is even more efficient.
I'm not trying to contradict Ed's experience, I'm in the market for a 150+lb self contained hammer myself, over a certain size the air requirements get hard to handle on a utility hammer. I just want to point out, that not all utility hammers are created equal. I've met some home made kinyons that were horrible, but if you think all utilities suck, and you haven't tried an Ironkiss, you owe it to yourself to check one out. The Bulls aren't made anymore, but I can crack an egg with mine or do huge single blows.
I wasn't as thrilled with the Big Blu I've played with, but it may not have been set up correctly.
FWIW, I've only got about $1500 total in both compressors, but they're used units, I had to replace the motor on one of them (I bought it without one). Having that air has opened me up to a world of useful pneumatic tools that I never could run previously, so I think that's another bonus.