Two AC 70s won't be enough. I have 2 AC110s on my 75. I'd go with your AC 75 and get a 110. The 110 is significantly bigger than the 70... I personally prefer HOBs over canisters but no one else agrees. They are too much of a pain in the to clean. I clean one filter each week and with my canister I hated it so much I only cleaned it every couple of months.
With the python, add the Prime and refill at the same time. I turn my python on then go and dump in enough Prime for the full volume of the tank, not the amount of water you're changing.