Definately buy everything you can, you never know when you might need those zombies, though after the third or fourth week you will rarely be in any real danger.. I think the only real problem a necro will ever have once he 'has the Force with him'
is an inferno army.. Against pretty much everyone else you can keep your troops, at least the ones that really matter intact, and in full numbers if they fall via raise dead. You can spirit link in the middle of a battle to regain your mana, this is a great perk so few people use, one that makes up for low knowledge. All is good if you don't face Inferno.
The reason is as simple as a three letter word: imp. As imps and familiars are very likely to act before any of your troops, you can kiss your already low mana, and therefore opening curses or raising the stack that was just wiped out by nightmares goodbye. There is surely a formula of how much mana imps steal but in my experience 100 imps will deplete around 50 mana before you can say dammit and expect to face armies with several hundreds in normal+ maps.. Spirit Link won't matter most of the time, for then you will be able to cast in the second turn, when -against a good player- nightmares had already massacred your troops.
As a matter of fact (and a little out of topic), I find carefully built Inferno heroes with light magic practically unstoppable in MP, especially when they have resurrection and lots of cerberi and nightmares.. A hard combination to have, inferno and light, but once you get it it is worth it... You can cast mass haste your already fast troops resulting in nightmares acting three or four times before most enemy troops, or cerberi and devil closing in without too much loss; or you can cast mass endurance to make up for low defence Inferno has (esp. works wonders for cerberi).. Couple it with leadership and luck, and you are on your way to be accused of being a "something that rhymes with mustard
", as I was accused in my last MP game, where I took out half of my opponent's fat army before the first turn with only cerberi and nightmares.
Really, Inferno isn't half as weak as it is thought to be and should be avoided like the plague by the necros in a MP game if at all possible. I would list the other towns' 'danger rating' against necros as inferno>dungeon>sylvan>academy>haven...