Have you considered the cost of accomodation, etc in Poland when attending the brush up courses?
Exactly! I think that's what many people forget when doing the equation, and it's not only about accommodation. First you need to get there (cost for airline ticket?) how are you going to get around (you might need a rental car) and of course, don't forget to eat. How many brush up courses are there?
Regarding your other question about time frame and study advice. Well, regarding the time frame, that's difficult to predict since a lot of factors come into play and each individual takes a different approach to learning/studying. But considering you're working a full time job, a realistic time frame in my opinion (from getting enrolled until passing the last exam) about 12 to 24 month. Of course there are always people who did it in less time, but I'd advise don't take that as a general guideline! The majority of the people underestimate the ATP exams, this is not an easy affair! It can't be compared with the PPL studies!
I'd suggest to use the search function of the forum, there are some helpful threads regarding ATPL studies and exams.
Good luck!