-Return to Dust, Swords, and Path can go in. I don't have Hero's Downfall but I do have Vraska's Contempt and Ruinous Path which I consider "close enough", plus Utter End is reasonably obtainable.
Any of those would be great. Ruinous Path is pretty mediocre in my opinion (I much prefer Never//Return as sorcery-speed Hero's Downfalls go), but it's still a totally solid option if you don't happen to have anything else on hand.
-Meren is mostly there because I wanted the reanimation engine, although it can be replaced with something more stable like Baloth Null
Consider Journey To Eternity. If you want to get spicy, Moldgraff Monstrosity is really fun to play with too.
-Lifeblood Hydra can go in although I'll have to buy it first. Is Genesis Hydra worth bothering with for this sort of deck?
I've tested Genesis Hydra in a few decks and I think outside of maaaaybe hardcore ramp it's stone-cold unplayable.
-I have Temple of the False God and Path of Ancestry, I don't have Exotic Orchard but I imagine Reflecting Pool is close enough.
Reflecting Pool is fine, might as well run it, but it's much worse than Exotic Orchard. I recommend anyone interested in Commander pick up 10+ Exotic Orchards, they can be gotten VERY cheaply online (just a few cents), they go in every non-monocolored deck in the format, and they're great trade fodder. I had 20 of them at one point and I'm down to a handful remaining between putting them in my own decks and trading them en masse to people who aren't already running them but have a bunch of decks to put them in. The rules change to Commander mana generation made them great, since they can now produce any color if any opponent controls a Command Tower. And they probably do.
-Cabal Coffers and Urborg, Tomb of Yawgmoth are right out unless I can straight-up trade for them. Sylvan Scrying can go in.
I'm not a big fan of Sylvan Scrying outside of searching Cabal Coffers or maybe other very specific utility lands in certain decks. I wouldn't just slot it in over something like Rampant Growth or Satyr Wayfinder or a mana rock unless you're looking for Coffers or Academy Ruins or another very crucial land. Some people like it just to search out Temple Of The False God though, which is fair enough.
Other generally good and inexpensive cards that are worth considering in your colors: Sunblast Angel, Graveborn Muse, Whip Of Erebos, Greenwarden Of Murasa, Woodland Bellower, Sepulchral Primordial, and Hallowed Burial. I consider all of those cards to be "personal staples", where I would think hard before building a deck in their colors that doesn't include them. Special mention goes to Austere Command, the best board wipe in the format in my opinion, but it's been creeping up in value for a while now and I don't think I could consider it "inexpensive" anymore.
Edit: Hooooly shit I had no idea Urborg and Cabal Coffers had gotten THAT expensive. I wonder if that happened after Hour Of Promise came out? Almost makes me want to sell mine, but my FNM Cabal Coffers has been with me for a long time and I've gotten tons and tons of use out of it, so I think I'll hang onto it. Also, I meant Deserted Temple, not Forgotten Temple, which is not a card. Incidentally, Deserted Temple may be my favorite Magic art of all time. I'd love to have a foil but they're monstrously expensive.
