I found the concept of proper level to be quite meaningless when DM'ing 3.5. Too many balance issues. Here are a couple tricks I found with crafting encounters in that system:
- If you have an item intended for the party as treasure, arm your npc with it instead of it being in a box. Nothing like a goblin wielding a flaming sword or a ring of true seeing.
- Diversify your npc's (ranged, mage, melee) instead of just having a single enemy type. Harder book keeping but better encounters. Honestly, that's one thing the design of 4th got right, even with all it's other flaws.
As for your haunted house adventure, mind games are crucial. A couple fun ones that I played with:
- Anything that creates an Escher (impossible) environment. Such as walking through the door of a room just in time to see someone leaving the room. Chasing the figure through an impossible number of identical rooms only to realize it was the same room and the figure was wearing remarkably similar gear to you. Perhaps inescapable unless a condition is met, something like a trap but more of a mental puzzle.
- If you really want to mess with your players and you have access to internet/speakers,
this ambient audio mixer might be fun. It contains a mixing board for randomly playing short audio tracks and it's got some neat horror appropriate sounds. Put it quietly in the background when they enter the house and you could put your players really on edge with the right audio choices and timing. Especially if they don't know it's on. :-) There are lots of good presets on the site but you'll probably want to tune it for the setting.
Good luck, sounds like a fun session you've got planned.