Woah, thank you all for responses I'm really surprised for the amount of help and feedback I've got! So I've looked at the market and it's really hard to find 25" frame in a sensible price. Most of the offers on ebay are from US and unfortunately shipping costs are always 2-3x times higher than the value of the frame itself. Therefore I must stick to the 21"-22" MTB frame or just hunt for some local offers here in Poland for bigger one.
Is it worth getting Reynolds frames instead of regular CR-MO ones? Is the weight difference/comfort noticeable? And somebody asked here for my weight I think I'm on the lighter side for my height weighing ~81kgs (179 pounds).
I might come up with more questions as my search progresses, thank you again.