First off...Welcome to the forum!
Since your question seems to be about filtration, I will address filtration.
I would forget the whole sump/undergravel filter idea. It is a good plan, no doubt. It is obvious you are giving this a lot of thought and consideration. Would your plan work? of course, its a great plan, but not a great plan for a planted aquarium!
As Justin has stated, the drawbacks of a sump and UGF are too great. I think you would out gas most of the CO2 you would be injecting. Is that an expensive problem, not really. CO2 is relatively cheap, but running around getting a bottle refilled is a cramp. It will be noisy, no doubt about that. You also run the risk of overflowing the sump and flooding out your place if there is a power outage. I am also not sure if a canister filter is going to be the optimal piece of equipment to return water from the sump into the aquarium.
The UGF can create problems for you if you plan on planting lots of Crypts and Sword plants. the roots of these plants work themselves everywhere ( I have had Crypt roots grow between flat pieces of slate and fishing line that was wrapped around the stone to tie down Riccia) and may inevitably get wrapped up into the UGF. Problem? Maybe. If you are the type that likes to move things around a lot, it will be a problem for you.
My suggestion. Get a canister filter. With a canister filter all of the above mentioned pitfalls will be avoided. If you are concerned with a heater in your aquarium, look for a canister with a built in heater (thermofilter) or an external heater that will fit inline with your canister filter's plumbing. Of course I would highly recommend an Eheim filter
, but the choice is yours.
Hope I helped!