Rank newbie, again.

I need to replace the stringers in a 3 step (not including top deck step).
This is mainly due to the fact they are rotting and falling apart. I've
talked to one of my lumber suppliers (no box stores like Home Depot or
Lowe's) and they recommended "treated" wood. Didn't sound too expensive,
$10 vs $14 for 2x10 (I need a 2x12). Anyway, is this a valid expense or
should I jes get reg wood (pine) and use a sealer (which I'm also woefully
ignorant of). I'm reading DIY sites as fast as I can. Again, not
completely retarded. I got a Skil mag 77 and know how to hurt myself!

nb