I'm personally in your same situation in many ways. I have one 30 minute 44cf SCBA tank. I'll be buying a used bigger tank, maybe a Drager 74cf or an Interspiro 88. Possibly even Omega 100cf tank. On the other hand, small, I'll be getting a regulated ninja pony type buddy bottle tank.
I've looked, and found great prices on used compressors from Ebay, and many other direct suppliers. I've found a very good compressor that starts at around $3K (that's a reps mark up included) add tax, shipping, insurance, oil, filling apparatus. You're going to spend at least 3K for any decent 3 stage compressor new and this brand comes with a full 2 year warranty. There's 3 things people do, or don't do that aren't good for compressors.
1. Most folks don't run it under no pressure for 5-10 minutes after a fill as a cool down cycle to reduce moisture.
2. I ran everything, compressors to hand pumps in a small room with a dehumidifier running and keeping it at 30% or less. Why let your filters do all the work? If you have a mobile type, run off your car battery, then I suggest a good inline filter like Joe B. sells
3. They don't run their compressor often enough. Even if you fill a big 88-100cf tank and it lasts you over a month. Run your compressor while not under pressure for 5 minutes once a week.
4. The old saying of if it has boobs or wheels, sooner or later you're gonna have problems with it. Add in 3rd stage on compressors. I'd say over 75% of any problems develop in the 3rd stage of smaller mobile type HPA compressors, as it's doing the hardest work. A 4 stage compressor isn't realistic for a one man shooting show. There's a 1986 50+CFM Bauer on Ebay right now for $1000.00. I don't know if it even runs, or what it needs to be rebuilt.
New, this was easily over $25K so a company like August Industries, or SCBAS, a fellow with experience, a machine shop, ties to Bauer could buy this, rebuild, sell to a dive shop or paintball field. In all reality, this is Military/dive shop grade area as even the 3.7CFM 3 stage compressor I'm buying has a 5 hour continuous run time. I will never be filling (6) 444 Cubic Feet cylinders to 3K and cascading. The Dive shop charges $12.00 per fill to go from 2K to 4500psi because they have to fill directly from compressor through control panel. Normally, they only charge $7.00 a fill of a SCUBA tank that's actually taking more air going from 0 to 3000psi. 500psi difference but compressor has to work harder to fill, so they charge more. The other thing I want to point out about my choice of compressor is it has the lowest RPM's of any compressor out there. 1050RPMs. A Bauer is around 1800RPMs, a Daystate Coltri, something like 2300-2500RPM's. Louder also.
I've decided I'm not running to the dive shop at least once every day and paying $12.00. 365 x's 12 =$4380.00 so if you shoot big bores, or a lot of anything, a compressor can literally pay for itself in 1-2 years, and the brand I'm going with has a 2 year warranty. I thought of trying a $1200-$2500 compressor. Nope, not taking a chance.
I will on the other hand buy the Chinese copy of a Hill pump as their tolerances are WAY better than The Crosman Benjamin pumps I rebuilt over 20 of. Heck, Hill just dropped from $287 to $199.00 Benjamin pumps are around $187.00 and a total waste of money. The Chinese Wally World 4500psi pump was $35.00, sold out and now $45.00, with 2 rebuild kits. Hill rebuild kits are $35.00 each. Point is, prices are coming down on EVERYTHING from hand pumps, rebuild kits, compressors of all types & brands as the air gun market increases.
So to original poster, if you're in the market for a good compressor but understand none of them are cheap, PM me and I'll point you in that direction. I'm not affiliated with them. I just have been researching and decided this is the best bang for buck, buy once cry once deal. I'll cry when I buy The Schmidt & Bender Field Target II also, and a Beaumont for it to go on.
Everyone has an opinion and ultimately what you do depends on your needs and your budget, so this is only food for thought. I chose to buy a good compressor (Omega) and two guns (I now own 5 PCPs) and not buy a tank at all. It seemed to me that the money I'd spend on a tank was better invested in a second gun. Having more than one gun also provides a backup if one needs repair and I could easily get in excess of 100 shots; more than enough for a days shooting. And as I'm only filling the guns with the compressor, there is less wear on it.
This is actually very wise in that he has less long run time on compressor. Yet running them often gets the oil up in it as needed. Just please do the cool down of 1-2 minutes if only filling up to 500cc rifles to get moisture out. Guys using Yong Heng to fill rifles, I get it. Filling 44cf bottles or bigger? I say expect your pump to die fairly soon.