Here is a link with pictures of one that is built exactly like mine:
Shop Made Air Cleaner - by Gene47 @ LumberJocks.com ~ woodworking community
My unit uses 3 filters that are 12 x 24 inches. The first two are inlet filters and can just be coarse fiberglass furnace filters.
The outlet filter should be one of the finer web type for filtration of very fine particals. Bottom slides forward or rearward to change the filters.
Not all that much to it. Just a fan,box, and some filters. Probably cost $100 to build 20 years ago. I see that Jet has one about this size for $500.00
YIKES that is a bunch of cash for a blower and some filters.

Good luck with your build.

Cheers, JR