pointlessly overly complicated sumps are pointlessly overly complicated....
yes you can add the homedepot pond filter to your sump... no it will not be enough on its own.. ponds and tanks are very different.. yes they both hold water and fish.. water bottles and toilets also hold water but are very different...
there are about 29845728957298357293 threads on this site about sumps.. if you can think of a question it has already been asked and answered about 50 times...
Nothing beats canister filters for ease of use. According to fluval, 1 fx5 is good for up to 400 gallons. This website is notorious for its over-filtration, so you'll get lots of advice on needing 3 fx5's and 2 aq110 hobs and a 25x/hour turnover rate.
If you're not feeding piranhas and don't have 100 5" fish in your tank 1 fx5 will cover your needs. If you do decide to overstock and/or feed extreme diet/amounts, you may need a little more filtration.
if you don't want to work or plan out a lot of stuff, get 2 fluval fx5's
if you have the time and energy to work, buy a 75 gallon rubber maid and make i into a sump. put a 20gallon trash can in it and make the lid into a drip tray. the water will come in and go onto the drip tray filled with poly fill or filter floss. then it will drip into the trash can filled with bio balls and then go into the main reservoir where you can put a baffle and make a refugeium or just pile it up with fluval foam blocks and ac foam blocks.
this sump does not offer a lot of mech so you might want to make an internal mech filter. use a 3 gallon plastic shoe box and cut 2-3 large AC blocks to fit into it. then drill a ton of hole in the lid to let water in. drill a hole in the side of the box and connect the hole to a 1200gph power head.
HERE ARE SOME PICTURES OF THE FILTERS YOU CAN MAKE. THEY ARE CHEAP AND EFFECTIVE.