I have a 2000 SSEI stock. Looking to put in a 3.6 pulley, 3" downpipe with a high flow cat, 180* Thermostat, Cold Air Intake and some new plugs and wires. With those mods how much hp and torque could i get out of it, how much would i spend, and would the engine last as long with the mods?

You should get 10 to 15 perhaps. hard to say for sure. That little bit wont effect the life of your engine at all.

then what bigger or better mods could i do?

Headers and rockers will open you up and should allow you to drop to a 3.4 pulley, but at that point I suggest you start scanning for KR. That should get you a good 25+ HP