Are there any mods that simply makes it so bandits, highwaymen, stay in their low-level armor instead of managing to acquire ebony armor/weapons and whatnot?

try francescos or oscuros oblivion overhaul. both feature that. fran comes in just vanilla resources and with a new items/creatures addon, welll OOO has a pile of new stuff.

I don't want new items or creature, though. I just want the highwaymen to stay realistic.

Check out http://planetelderscrolls.gamespy.com/View.php?view=OblivionMods.Detail&id=268 then, i believe you can go with only the overhaul and no new items, creatures addon. id also look into T.I.E., as it utilizes only vanilla resources as well.

I'll give it a shot.