Well to answer your questions;
Hardware: Step/Direction driver, many available from the more expensive (Gecko) to the cheaper ones you find on ebay. I would recommend http://www.routoutcnc.com/stepperdriver.html but what you use depends on the stepper motors you have.
Software: Mach3 or emc2.
Mach3 is from ArtSoft, reasonably priced and well supported. emc2 is available free under the GNU GPL.
Now, you say to cut and engrave PVC... bad idea!
PVC is PolyVinylChloride, which when exposed to the laser will produce Chlorine gas (the green fog that kills) and when combined with moisture in air (and your lungs) will convert to Hydrochloric acid.
You may have read about laser cutting PVC, yes it can be done if you have the correct extraction system but this isn't likely in a hobby environment.
I would suggest Acrylic as an alternative, or another suitable thermoplastic.