https://deckstats.net/decks/112190/1051092-muldrotha-the-gravetide-perman/This is the Muldrotha deck I've got. It was interesting to build because I limited myself to 100 permanents, which means I can get everything back. Take a look on it. Some cards may be out of budget for you, but you may find some interesting ideas.
About
Spark Double: Of course you'll need the mana to cast the cards, but it allows you to abuse some cards even more. For example you could play
Sakura-Tribe Elder twice for 2 extra basics. Same goes for the Seals.
The point is, that you're able to cast 2 cards of the same type, which doesn't mean you'll HAVE to do it. This gives you some flexibility. Besides that, The
Spark Double isn't expensive (mana- and moneywise).
In general I'll usually try to put just enough cards into my graveyard that it would justify a hate piece to clear it. This may make the deck itself a little slower, but a single graveyard hate can't break your neck. Because of this I prefere cards like
Millikin or
Monastery Siege (Kahns) that slowly but reliable fill up my graveyard.
Sultai Ascendancy is very useful here, since you're able to control what goes into your graveyard and what stays. This is especially important if you have to deal with permanent graveyard hate like
Rest in Peace.
/edit: if you're afraid of graveyard hate,
Elixir of Immortality does a good job and is reusable, since it shuffles itself into the library, in stead of going into
exile. But on the other hand, it doesn't mill.