Started my second character last week, almost level 20 so far from simply exploring/dungeons alone. Khajiit Assassin.

I'm afraid of leveled loot and items becoming pointless. What's a good level to do these questlines so i'll get valuable gear from it?

The nightingale armour is highest at level 32, although the weapons are higher (40 something)
not sure about the DB armour though sorry.

Level 35+ I guess.

Higher if you want the sword maxed but I've heard you can't upgrade the maxed sword properly so...

Nightengale is thieves guild right?

yeah, you get it further down the quest-line. you get the standard thieves guild stuff too - a word of advice, if youre not going to use it, dont sell all the pieces, keep at least one, youll need it.

ive started as soon as i created my person you at least need to be like level 15 im dying in like 3 hits so wait a bit do all the side mission for vex and the other guy twice to get some cash and armor upgrades

at level 20 you should be ok, especially since ideally you wont be killing as much as sneaking.

According to the wiki the DB items don't scale but the thieves guild rewards do. The weapons max out at 46, the armor at 32.

Source: http://uesp.net/wiki/Skyrim:Leveled_Items