Well, the bible is Dolf'sbook Grand old Lady... but it is out of print and very expensive. there is a newer book on the vickers, I got it off Amazon. If I can find it I will post it but you should be able to find it. It is better than nothin and a lot cheaper. the balloon gun you refer to is not one mounted in a balloon but used to shoot them down from aircraft. It fired a 11mm incendiary bullet. Realistically you will be looking at remanufactured transferable standard ground gun. what comes with it will make the difference-
Personally I have Browning 1919s, 1917A1s, Vickers, 1910 maxims, 08s, 08/15s and a raft of other belt and strip fed guns. For ease of use and portability, the 1919A4 is probably best. the points of ammo and parts are valid, but realistically I rarely break any parts. Look at ammo cost, conversion cost, ( what cals can a gun be converted to for how much) I shoot my Vickers in 54R because I have a LOT of cheap ammo . If I had to buy the ammo today, I would probably reconsider the cost to convert it. The water cooled guns are a bit more work to take shooting. The flip side is they shoot all day as long as you keep water and ammo in them. You need t really considwr how much and where you will shoot it.