I personally wouldn't worry about it being fish-safe once you thoroughly cleaned it.
The drawbacks as I see them:
1. In order to service it you'll have to drain at least half of the water out.
2. Due to the "hamburger" nature of the container, your media-space is limited to what you can pile in there with it open (maybe a hair over half-full?)
3. pump, media, plumbing parts and accessories will cost you more than a purpose-built canister. (Unless you already have the parts laying around)
4. PITA.. Will it work? Probably.. But the difficulty of servicing the device would make me less likely to do that servicing. I have a cannister crammed under my 21" stand with literally 1/8" clearance when the hoses are attached, you have to tip it (which makes a mess) to attach or detach the hoses. I hate servicing that thing, and so I do it less often than I probably should.
Benefits:
1. Fun! I love DIY stuff, and it would be fun for me to build something like this
2. Unique. Who else has a military grade plastic canister?
3. Pride. I like knowing I made something that works.