1. Johan is rarely a bad choice, but from there its all master specific. Molly likes Killjoy and McMourning might want Performers. Tara Yan Lo might want models from Outcasts and 10 Thunders, but I've never played them (or against them).
2. It depends greatly on which masters you're looking to play, which upgrades you want to give them and how you'll be playing them. Seamus might want more than 3 Belles (but it would be rare), McMourning could very likely use more than 4 Canine Remains (1 from his box, 3 from the minion box) and definitely 1-2 additional Flesh Constructs, Nicodem (especially with the Emissary) might want 2 boxes of Mindless Zombies. You cant buy seperate Seishin, there's 2 in Kirais' box and 3 in Datsue Ba's box, speaking of the spirit ladies, 2 boxes of Gaki might be usefull.
A general piece of Malifaux purchasing advice: If a master box contains the minions your after, said masterbox is almost always a better buy than a seperate minion box. Some master boxes might even be worth buying even if you have the master, since you can sell the master, totem and henchmen online, as these arent sold seperately (this might not be true these days though, but there were a lot of gaps in 1st ed. players collections in the beginning of 2nd ed.).
Unless you really focus on it (be carefull not to forget your strategy and schemes) you wont be summoning all that much, so as others have said above: start with one of each