I too am a graphic designer with basic web skills,
So, I thought I'd just let you know, I've just done this, (setting powerstore 3 up with paypal pro) and yea.. it's not to difficult.. it took me a while (and one call to Paypal) to figure out exactly what the API credientals (Signature) were, and where to find them, but it was all ok...
With regards to Powerstore 3 in general:
You will inevitibly come across problems when you build the store and there will be tiny little things that will bug you and take a whole day the first time you do them (and an hour the next time you do them!) (But realistically that's no different from learning, Photoshop, Illustrator, InDesign, Dreamweaver, Flash, etc..) but the webassist forums are good, and webassist staff (an other members of the forums!) are usually pretty good (and quick!) at solving problems!..
it won't be as easy as you think it is, (or quite as easy as webassist videos make it out to be, sorry guys, just being honest!) and you won't get it done as quickly as you think you will, but ultimatly with these tools and this forum you'll get it done!.. ;-)
hope this helps!..