The very best M2s come from Live Resin in Russia, available direct at www.models-hobby.com or from various resellers. He does the 1940's-1990's M2HB and the current M2 QCB in packs of 3 guns for about $8/£7 (at today rates). I've just ordered half a dozen packs today. He also does various modern and wartime mounts, ammo boxes, belts and loose rounds.
The associated company Tahk (Tank) used to do excellent resin M1919's and both early and late types of mount. But these are long OOP and very hard to find now. Believe me - I've been looking! ET Models do an etch set for 2 M2's with mount, ammo box and holder etc (EA35-003). They also do a set with a nice resin M2 and the etch set (EA35-040). Eduard used to do a set of 3 early etched mounts for the M1919 (35030) and 2 mounts for the M2 (35031), which may still be available .
Voyager do a couple of Israeli MG sets for Tirans and Shots which have an M2 and 2 M1919's in resin plus mounts, boxes, etc (try VBS0123). They're £9-£10 a set.
Check out the Zebrano resin M2's, which come in 2 different mount types. They're not bad at all for about £4 each. They also do ammo boxes in a couple of types and ammo belts.
Beware M1919 sets where you have to roll your own barrel jackets from a flat etched sheet and attach a wire barrel and resin muzzle. Nightmare. RB Model do very nice brass M1919 barrels that you can pick up for as little as about £1.70 each. They also do M2 barrels, but the perforated barrel shroud is a little basic and maybe a bit short. The Orange Hobby ones are good and a reasonable price. You can pay £10 a barrel for some brands.
Panzer Art have just released some extremely nice 0.30 and 0.50 ammo boxes, possibly the best yet. Etched ones from the likes of ET are good for showing opened or crushed cans, open and discarded lids etc but are damned fiddly to assemble.