120 can probably get you a decent sub, probably used. Now, do you already have an amp to power them with? If so what kind/specs. If not, keep in mind the more an amp puts out, in GENERAL, the more its gonna cost (not counting a killer deal you may find in the FS section). So you'll want to keep that under consideration. You could probably look at mach 5 subs for that price, and start saving up for money for box supplies, and then a decent 500 watt or so amp. Or just continue saving and get one of SSA's sundown/sub combo's. Then have fisher build you a box for it. Heck, you might even be able to swing a sweet package deal.
Just some food for thought. FI's are good subs, as are sundowns, but plan it all first before you go dropping hard earned coin.