I am not exactly sure what your price range is but so far I can assume it is between $0 and $7
How about these?
Necessary
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Geth, Lord of the vault //Mill and more
Lich Lord of Unx //Mill
Unholy Grotto //Tricks and Recovery
Undead Warchief //Add to damage/mill
Cabal Interrogator //Removes enemy threats and feeds their graveyard
Gutless Ghoul //Feeds yourgraveyard
Fleshbag Marauder //Feeds your and their graveyard
Skullclamp //Feeds your graveyard and draws crazy amounts of cards
Carrion Feeder //Feeds you graveyard
Balthor the Defiled //Resurrects your graveyard
Patriarch's Bidding //Resurrects your graveyard
Noxious Ghoul //Feeds their graveyard
Stronghold Assassin //Feeds their graveyard
Altar of Dementia //Mill and feeds your graveyard
Tombstone Stairwell //This card is tricky (see below)
Call to the Grave //Feeds their graveyard and keeps you alive
Strongly Recommended
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Zombie Trailblazer //Wins games
Filth //Wins games
Bone Dancer //Situationally awesome
Levitation //Wins games
Plaguebearer //Kills tokens and utility creatures like mad
Vengeful Dead //He's scary
Nantuko Husk //Feeds the graveyard
Withered Wretch //Removes pesky graveyard stuff - depends on your meta but VERY good
Convenient
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Cackling Fiend //Meh, he's OK
Corpse Connoisseur //Feeds your graveyard
Twisted Abomination //Feeds your graveyard
Jhessian Zombies //Feeds your graveyard
Undead Gladiator //Feeds your graveyard
Wow, that was a lot! I want to make two further points; the first is about Noxious Ghoul. Noxious ghoul isn't all that great in the normal play of an EDH deck unless your opponent is playing really small creatures which probably isn't happening. However, when you use cards like Zombie Apocalypse, Patriarch's Bidding and Balthor the Defiled, Noxious Ghoul takes on a new role. When he comes in to play with all those other zombies his ability triggers for all of the zombies that returned at that time. In many cases this is a blow-out because you now have 6+ zombies and all of your opponents have nothing.
The other point I would like to make is about Tombstone Stairwell. This card has a lot of really annoying text on it but what it basically says is this: "every turn, every player gets a 2/2, hasted zombie for each creature in their graveyard". That has the potential to be really dangerous for you so you really have to know when to play it. Here are examples of when to play Tombstone Stairwell:
1. You have more creatures in your graveyard than anyone else and one of the following:
Levitation
Filth + they have a swamp
Filth + Zombie Trailblazer
Vengeful Dead + (Carrion Feeder, Nantuko Husk or Grimgrin, Corpse-Born)
2. You have at least 1 zombie in your graveyard, can likely survive the round and Altar of Dementia in play. Spoilers, you're about to win the game by milling yourself each turn until you have several zombies in your graveyard and then start milling your opponents.
Don't worry about the cumulative upkeep on it either. If you're in one of the scenarios I outline above you won't need more than a few rounds before you win (or get teamed up on by 6 people because they know they will all lose if they don't ).
I hope some or all of that helps make your deck better while staying in your budget. If I could recommend a single card out of your budget it would be Gravecrawler because he is simply broken with Grimgrin and many of the other effects I just mentioned.