Alpha Wolf

The heaviest it is able to hop is generally best. You will have to play around with a few and see what works for you, hop and range wise. 

Again with mags it depends on you, how many are you able to carry and now many shots do you need? 

I try to take as little out with me as possible so I use high caps, I have seen others goout with 8-12 mags on them (although I never understand why). Take as many as you feel you need. As with 95% of airsoft its all personal preference. 

plenty of midcaps is good if you hate the rattle/winding/general hi-cap thing.

depends on your playstyle as to how much ammo you use, the site (for example length of games, reload opportunities, honesty of the playerbase etc).

for a sniper the aim is range and precision without volume, so the heaviest and best quality ammo you can get your hands on would be the reccommendation, of course the gun may not be able to hop the super heavy stuff so you can either run lighter or get into upgrading and get the hop able to carry the heavier bb's.

you'll likely be able to get away with just the 1 mag and a speedloader to top-off in the field.

It's not like you're going to do speed reloads on a sniper rifle.

The supplied mag + a speedloader is how I started playing 11+ years ago.

With my current bolt action I carry a whopping 2 mags (one of which in the gun). They do hold 45 bbs each tho