Be super super super careful when buying land in Poland. There many crooked unethical people in this industry that have screwed over foreigners - both people who are from another country as well as Poles who have lived abroad and returned to buy property. I'd highly recommend hiring a lawyer to help you and checking every step of the process, redundantly documenting everything, and basically just make sure you're totally covered from all aspects. I've heard quite a few horror stories of people getting screwed over and losing tons of money due to unethical agents, title companies, etc. Just be very very careful. It sounds like you're doing your research though so you're off to a good start - keep it up! You'll love Czestochowa - it's a very well known town in PL and very pretty.
I'm being told I cannot sell the land for 10 years after purchase, as a small part of the total land is classified as 'RZK' (Farmland and place of natural beauty). Is this correct?
To my understanding, yes that is accurate as the land I own in Poland too is classed as agricultura.