Blocks run from about $100 to $200. To me that isn't really all that expensive when you consider what other AR stuff can cost. The blocks themselves have to either insert from the top or the bottom of the magwell. Inserting one from the front back or side just isn't an option. The mags will insert from the bottom, just like on a regular AR. I am not aware of any blocks that require the mag to be inserted from the top.
For Uzi mags, you can use Hahn, RRA, ASA, or Garrison blocks (require modified Uzi mags), or the Hytech block (uses unmodified Uzi mags). There are also other blocks for different mags.
Uzi mags need to have the notch at the back opened up and a new mag catch notch cut. I did mine at home with a dremel. Out of 15, I only had one (the first one) that gave me any problems.
No need to replace the mag catch.