Heh, multis dig, but not to bury their shells like some other shelldwellers do. Instead, they try to get rid of the sand, and will dig all the way down to the glass where they can.
As for plants, I'm always a fan. I'd probably stick with fern, moss, and anubias, which can all be anchored to wood or rock, and don't require ferts or CO2.
I've had both snails and cherries in this tank. The multis will pick up snails, swim them out of their territory, then drop them. Both males and females do this, though males, being bigger, will pick up bigger snails. They may only do this around females shells.
Likewise, they seemed to treat the cherries more as intruders than food. They'd attack them when they found them, but weren't particularly bothered. I made the mistake of putting in ghost shrimp which were a bit bigger, and the multis freaked out.
I think your endlers will be fine, but will be harassed some. I'd give them some cover in the form of fern or moss, or maybe floating plants like riccia or water lettuce. Those will also keep the algae down, I think.
Maybe get 50 shells or so. That would be $10 + $8 shipping. Pile the shells up on the sides and back, sloping down to the center, to create a little bowl effect. You can see how your multis behave with the sand, and also see them navigating the maze of the shellbed on the sides.
I also have some japanese land snail shells available, some of which have been in my tank already. The multis seem to like them, but I've read messages which caution against using shells with a spiral because the fish can get stuck. None of mine have, but I've only had them a month or so.
The japanese land snails would also be 5/$1.00, but I haven't figured out what shipping would be. They're a very light shell, so I imagine it would probably be comparable.
japanese land snail shell sizes
japanese land snail shells in the tank