Shop the auction sites. You can still find some Belgian A-5s.
I've never been too much of a fan of the old spring operated (not gas) autoloaders, such as the Browning. Point of fact, I've never been too fond of semi-auto shotguns as a whole.
For competition, I'll stick with over/unders. However, for hunting and collecting, I love my side by sides.
If you're looking for water purification, why not look into rain catchment and a cistern.
If we were younger, we could dig it out, tile/brick it, in a few days.
Now, look at it as a two hour a day, two month project.
Although, for around $1200.00 (stimulus bribery check money), you can get a plastic 3000 gallon water storage tank.