I have the same mix of belt feeds as you along with the Vickers and a 1917A1. The Vickers goes with me on most shoots.
1. are they fun? - yes, a different operating principle than a BMG, but not as heavy as a Maxim. It has a cadence and unique sound to it just like a 60.
2. are they reliable? - very, and easy to tweak, too.
3. what's the best caliber to shoot them in? - I like .54R because it is easy to load belts with a PKM loader. As a rimmed round, extraction is very reliable. The lock is easy to modify, and for the barrel, just ream out the Vickers .303 barrel. If you shoot .54R, don't forget to take off the booster. Recoil impulse from this round is strong, so not needed or it will beat up the gun.
4. what to look for in a shooter grade Vickers (I am not interested in a collector grade gun ) - VSM, Erb side plate guns are good. Fluted water jacket if you can find it.
5. mandatory accessories? - Several locks, firing pins, V spring, PKM loader
6. replacement parts supply realm? - readily available, better than a Maxim
Good luck. Vickers are well priced now.