For the LVL ? Death spell, you can hold \ (on default controls) to look at the scoreboard and see enemy levels, or you can just click on the enemies to check. If you're just taking on one guy though, you might as well blow the LVL ? Death on him no matter what level he is, as some damage is better than no damage.
As for the other spells, read the description on Devour again. You gain the 2 abilities that creep you devour has. The key here is that it applies to neutral creeps! You om nom nom one and you get it's abilities. Also, remember if you have abilities from a creep, and you eat one that has no abilities, you won't lose the abilities you have, so don't worry. However if you eat one that does have abilities, then it will replace the abilities you currently have.
As for which neutral creeps to get, I personally like to get these guys:
They have an ability called Ensnare, which is essentially like Crystal Maiden's freezing thing (They can't move, but can still use abilities and items). It's pretty damn useful to be able to hold them down like that when you're chasing. Getting the Centaur is pretty good too, as that stomp would help you if you can get up close and personal with the enemies.
Key thing about Doom Bringer though: Om Nom Nom is a priority early on! He's a carry, and that's a great tool for farming! I pretty much do something like this for him normally (in order of priority).
1. Devour
2. Scorched Earth
3. Doom (take ult as soon as possible, level every chance you get)
4. LVL ? Death