Having been playing this heavily for 2+ weeks now, I can give some thoughts on my experiences. I also started with necro, and had to replay some of the campaign missions multiple times before winning. The necro starter deck is probably the weakest of the three due to very poor spell selection and weak creatures. That said, after the addition of some of the great necro commons, necro is probably the best against other starter decks. Necro does struggle again at the very high end since the top decks are either extreme rush, or extreme control, neither of which does necro do well.
As for what to buy, first of all absolutely do not waste your wildcards on anything other than crafting epics (orange cards). Crafting a specific hero with wildcards is ok, but make very sure it's the hero you want since they are expensive. The game is a bit of a trap in that when you start, you get flooded with a ton of gold/seals through one time achievements, beating the campaign, and fast leveling at the low end, which may lead to spending them without much planning. Thing is though, after that early rush of rewards, it slows down dramatically and wildcards / purchases come much less often (unless you spend cash).
As a necromancer, your first wildcard purchase should probably be
Atropos, unless you happened to get him from a pack. He's a staple in just about every necro deck. To get the best value for seals, you want to buy the champion box first, but it costs 3000, and earning 3000 seals with only the starter cards would be extremely hard, so that's only really viable if you spent some cash. Just "The Box" is a common first seal purchase to get a base of cards to start with.
For gold, as a necro, you probably want to buy packs from Herald of the Void. One of the best necro creatures is
Decay Spitter, and
Soul Consuming Lichis a great frontline blocker that is hard to kill.
Adar-Malikis one of the strongest necro heros as well. All of these cards come from Herald of the Void packs.
Edit: Another option as necro is to save up 125,000 gold for the pre-built Arrianna deck. She's a rare necro hero, and her deck comes with a few great cards like puppet master and soulreaver, along with just about all of the base set standard creatures you need like vampire knights and neophyte liches. If you didn't spend any gold, you could afford her deck by around level 7-8ish.