I'll save you the trouble of PM'ing me and just answer your questions here
Yes, the ACM 6mm M500 is an awesome gun and should last you many long years of use with little to no problems. But... only if you're willing to do a very small amount of initial internal tuning up.
First off, the charging arms need to be replaced with steel ones. I used Private Parts steel charging arms that can be had from US based retailers like AEX and Airsoft GI for around $25 I think. These may require a small amount of material dremeled off near the front of the charging arms to fit the forend mounting bracket correctly, but it's really no big deal if you've got a dremel tool and a grinding bit around. It'll take you all of 10 minutes or so.
The other thing you'll have to do right away is lube up or replace most of the o-rings in the gas tank assembly, there are 5 of them total. Fortunately the ACM M500 series comes with a spare bag of replacement o-rings floating in a little bag of oil. There's a replacement for every single o-ring in the gun in there, so if any of the o-rings in the gas tank assembly appear dried out and cracked or are leaking they can be replaced with an o-ring from the bag of spares included with the gun. Or if you want to get really fancy and buy o-rings that'll last for just about forever you can pick up some Viton o-rings.
Lastly, when putting everything in the gas tank assembly back together I recommend you wrap a good 4 or 5 layers of PTFE tape around any part that has threading on it to insure a good leak free seal.
In short, it'll leak right out of the box and the charging arms are pretty crap but if you're willing to take a little time to take the gun apart (which is extremely easy and fairly newbie friendly I might add) and replace the charging arms, replace or lube any o-rings that need it, and PTFE tape all the gas tank threads than you'll have a gun that should last years and years of regular use with no problems.
As far as on the field performance goes, I skirmished with only a ACM 6mm M500 for an entire season (last season actually) and really enjoyed it. If you plan to use it at MAA games it will be chrono'd with 1 BB in the chamber (even though it fires 5 BBs per shot every shot) and if ran on duster gas it will produce around 430 fps with one BB in the chamber. On propane or green gas it will produce about 550 with 1 BB in the chamber, so if you want to use it at MAA games you'll need to run it on duster gas only.
With duster gas and 0.2g BBs (shooting 5 BBs at once as per normal operation) your effective range will be about 80 to 90 feet. Not bad considering you're lobbing 5 BBs about 270 fps out of a gun with no hopup at all.
The 5 BBs will produce a spread about the size of a baseball at 20 feet, a softball at 40 feet, a basketball at 60 feet, and by the time they get out to 80 or 90 feet they blossom out into a roughly man sized spread. This is extremely realistic when compared to the effective range and spread of a real 12 gauge shotgun if loaded with 00 buck, so the realism of range and spread are pretty spot on to the real deal.
When using green gas or propane the effective range is around 110 feet with roughly the same shot pattern progression as on duster gas, the shot just stays a little tighter a little further out.
As far as capacity and gas efficiency goes, the gun holds a total of 180 BBs in a 3 tube internal magazine system (60 BBs per tube) and since it fires 5 BBs per shot you get 36 shots before you need to reload. The internal gas tank holds a ton of gas, enough for about 45 to 50 shots if you allow 3 or 4 seconds between shots to avoid cooldown. If you rapid fire the gun (racking the pump and pulling the trigger as fast as possible) you'll only get 20 shots or so before the gas runs out due to cooldown, so take your time and pick your shots.
If you skirmish a lot of CQB or you don't mind being a little sneaky and stealthy in outdoor games than the 6mm ACM M500 should serve you just fine. In the season of skirmishing I used mine in (all outdoor games) I got about the same number of kills I usually get with an AEG, I just had to change my tactics a bit and be a little sneakier to get in close enough for the kill. And you learn to do that pretty quickly once you realize your range handicap
The only thing I'd recommend picking up as far as accessories or maintenance stuff goes is a VFC portable gas charger and a Airsoft Innovations duster gas adapter since you'll need to run the gun on duster to be MAA legal. The VFC portable gas charger is a little bottle that looks like a grenade but can be filled up with gas through a valve in the bottom, then the spoon of the grenade can be unscrewed to reveal a hidden nozzle which allows you to fill your gun with gas from the fake grenade. The advantage of this is that the grenade holds a lot of gas but is small enough to carry around on the field easily, allowing you to top off your gas reservoir while on the field if needed. And the duster gas adapter, well it goes without saying what that is for but you can also stick it on top of a silicone oil spray can and use it to inject silicon oil directly into your gas tank every now and then to make sure your o-rings stay well lubed and never dry out. That's really all you'll ever have to do for long term maintenance though.
Finally, as far as cosmetic accessories go the ACM 6mm M500 can take real steel Mossberg 500 forends with zero modification required (just remove that little silver clip and screw on the inside of the forend mounting bracket before installing the real steel forend, they serve no real purpose) and they can take real steel Mossberg 500 stocks with minor dremel modification to the stock.
Hope that helps!