-Anything I need to know?
Yes. Plan your route carefully using as many cycle paths as possible. The low driving standards here haven't improved however the number of cars has increased hugely.
-What is the quality of bike shops, and (if possible) a rounded price quote.
I'm not sure of the quality, however most big towns and some smaller ones have specialist bike shops. There's probably a Polish-language forum for cyclists that lists the good ones, given how popular cycling is here. For spares/supplies, most shopping mall sports shops will stock at least the basics.
-Are bike thefts still common? If so, are there good, safe places to store a bike
Yes and yes-ish. The bike racks outside shopping malls are relatively safe, and have cameras/guards nearby. Nevertheless, thefts do happen. Bring/buy a good bike lock.