If you really want a successful "low tech" tank than you must abandon terminology such as "upgrade". Save that for the "high tech" tankies. The ultimate goal of low tech would be minimum filtration.
Aqua Clear Filters should last you many years. Perhaps a Tuneup is in order. have you tried clearing the intake tube? Also disassemble the motor by pulling the pin that supports the impeller and clearing it of any crud. AC's tend to run better the older they get if maintained periodically. They seem to get worn in and become absolutely silent after a year of run time.
I regards to CO2 Bruce is right. You do not need to worry about water movement. However let me tweak this by saying... You should worry about a lack of water movement. Particularly at the surface. Unless your adding CO2 manually you are going to be dependant on CO2 from only two other sources. Your fish and the atmosphere. Your fish are not going to produce enough to keep the plants happy.
The good news is that unless you running high lighting and adding nutrients, there is plentiful CO2 in the air that we breath. Just look around the world, lots of healthy growing plants everywhere you look!
To take advantage of all the "free" CO2, you want to maximize you "gas" exchange with the atmosphere by creating surface agitation of the water and allow the atmospheric pressure to "equalize" the amount of CO2 in the air with that in your tank.
Through careful study, and patience I believe that you could set a goal of having this tank without water changes. On top of that you should be able to do it without filtration. However you must be willing to take this venture seriously and question everything everybody is saying, especially me.