In Fallout Shelter, is it better to send max equipment Level 1 dwellers over Level 50 into the Wastelands?

No, not really. As you already stated, the health of the Dweller always resets when they level, that means they will already last (roughly) 50 times longer, even without the Stimpacks.

The downside is that you only have a finite amount of low level dwellers, but an unlimited Stimpack production. The other catch is that every Dweller, regardless of their level, has a finite amount of storage, So they will return once their carry weight has been reached, regardless of level.

This is all up to chance though, too. There is a chance they might not get enough experience to level up properly, and not find any loot, so they may still die. That chance is fairly small though.