I run 2 canisters on my 4 foot tanks. My tanks have a turnover rate of 8 - 10 times an hour, regardless if its a HOB or canister. I too don't clean my canisters more than a few times a year, max.
I have renas, fluvals and sunsuns. The sunsuns are my favorite, and there is no beating the price. I am actually replacing the fluvals with them. Sunsun are the white filters sold on ebay. They are either sold by the name sunsun or perfect. For your tank, I would get a 265 gph model and a 369 gph model. Cost of both, with shipping - less than $150.
If you add duckweed, you'll NEVER get it out, and will possibly spread it to other tanks where you'll NEVER get it out. The leaflets are just so small that even when you think you've gotten all of it out, it comes right back. I strongly suggest not getting duckweed.