I'm not sure what "wealth per level" is, AD&D lacks such a concept. Dungeon level one contains monsters drawn from the ranks of encounters suitable to level one PCs, yes. A small percentage of those may be stronger than average. IIRC the DMG dungeon encounter tables include a small percentage of level two and three monsters.
Good DM practice in terms of placed encounters is that they fall into these general difficulties. If a particular encounter area is unusually difficult or presents a unique challenge then this is a discoverable circumstance. Rogues should be scouting, maps acquired, rumours sought out, guides employed, etc. At the very least parties employ stealth, listening, searching, environmental clues, etc. This is all spelled out in various books and such.
If a GM employs some form of gotcha or unusual situation then either it simply ratchets up the hazard, acts as a more narrative obstacle, or has some fairly reliable (perhaps costly) way out, etc.