I'm not a big fan of python gravel cleaners but you can use a gravel cleaner like the python to drain water out of the tank and into a drain that is lower than the tank (on the floor). Just let gravity do the job for you. Then use your pump and attach that to the python to refill the tanks, or get a separate hose for that.
Powerheads, sump pumps and water pumps are all the same sort of thing, a submersible pump that moves water. Powerheads tend to be smaller than sump pumps but they all work off the same principle and usually have an impellor that consists of a magnet with plastic blades on one end. This sits inside a plastic case that has an electric coil and some electrical components. The electronics are sealed in a plastic resin so water can't get in. Water is drawn in one side and pumped out another side.
Wattage is irrelevant when it comes to water pumps and the only reason you look at the watts on a pump is to find an energy efficient pump to reduce power consumption. If your pump is only running for an hour or two each week, when you do water changes, it won't make a lot of difference what wattage is used by the pump.
Because the water being pumped into the tanks is on the same floor as the tanks, you only need a water pump that can pump up x amount of gallons at the maximum height of your tanks. You need to measure how high the top of your tanks are from the floor, and that is how high the pump needs to pump water. So if the top of your highest tank is 5 feet above the ground, you will need a water pump that can pump 50 gallons per hour (or more) at 5 feet. If you get a pump that does 100 gallons per hour at 5 feet, that would be fine and fill the tanks quicker than a pump pushing 50 gallons per hour. However, this might be a bit quick for small tanks.
I used an AquaClear 800 powerhead to do this in my place. The top of my tanks were 4.5 ft above the ground and the powerhead did it easily. I just had some clear plastic hose on the outlet of the powerhead and made a U out of pvc pipe to go on the outlet end of the hose. The U was hung upside down over the edge of the tank and allowed me to leave it hanging on the tank while it filled up. When the tank was full, I turned the pump off at the power point, moved the hose to another tank and refilled the holding bucket (containing the powerhead) with dechlorinated water. I then turned the pump back on and filled another tank.
A powerhead or sump pump or pond pump that pumps 50-100 gph at 5 feet would probably be suitable for you.
Get a brand name pump (Eheim, AquaClear or something common) and that has a decent warranty. Most good pumps should have a 2+ year warranty. Some of the cheaper pumps won't have any warranty or might only have a 6 month warranty. You can try the cheaper pumps if you like but you generally get what you pay for. My preferred powerhead/ water pump is AquaClear powerhead and then Eheim Hobby pumps. There are plenty of other brands so find something that is suitable for your situation and where you can get replacement parts. The main replacement parts in these things is the impellor, impellor shaft, and rubber gromets that go on the ends of the impellor shaft. Most companies sell these items separately or as a unit (impellor, shaft & gromets). The parts don't normally wear out that quickly but the impellor shafts can be made of steel or ceramic, and the ceramic ones can break if you handle them roughly or drop them. However, the ceramic shafts don't wear as quickly as the metal shafts do. Most pumps have a metal shaft, Eheim is the main one that used to do ceramic shafts.
If you find some pumps online, post a link and we can check it out. And tell us the max height of your tank so we can try to match something up.
