I'm collecting items in Skyrim, and I need to know if low level items, such as minorly enchanted weapons/armor (Blessed Iron Dagger, Steel Battleaxe of Dismay) or base unenchanted items (Iron Sword, Leather Armor), will start to become very rare if not dissapear at higher levels.
Shop keepers do not end up with an entire inventory of Daedric/Glass equipment when you reach high levels.
The list simply expands, with bias towards low/mid range, with the rarer items still actually being rare.
Before a certain level, they simply will not exist. The Paralysis enchantment for example. It is level locked. However, after you reach a certain level, it is simply available, but is still rare. Shop hunting for that Water Breathing enchantment is something you do have to do if you want it bad enough.
Even at late game, enemies are not loaded to the brim with top tier gear (unlike Oblivion... where random Bandits will have couple pieces of Glass gear).
Also, not all areas scale fully to your level. The starting area's bandit cave will not, for example, contain Daedric stuff just because you reached the appropriate level. It has a maximum level that it can scale to, so it limits the quality of items it can contain.
Mostly no, but watch for two specific enchants
According to UESP, the muffle and waterbreathing enchantments are difficult to find at low levels and nearly impossible to find at higher levels. This is especially annoying because these enchantments happen to share an unusual property: enchanting them always produces an item of the same potency regardless of the soul gem size. This makes them useful for enchanting junk that you plan on selling (because you can do it with petty souls instead of having to burn grand souls for maximum profit).
Skyrim uses levelled list to match the loot to your level, so citizens upgrade content of their pockets from garnets to diamonds, but not every list of loot is levelled one - I still remember finding iron daggers and leather armour in chests at higher levels, but indeed it was little bit more rare - if chest had a chance to contain a weapon, at lower levels it would be anything "iron", at higher one it would be selected from much bigger list containing anything from "iron" to "deadric".