Filtration: what would you do?

If you add an additional filter to the Eheim you have now, you have the advantage of keeping one filter operating while maintaining the other. You could also put them on separate power strips with circuit breakers, and if one fails for some reason and shorts out, it doesn't take the other one with it.

What is the actual GPH for the Eheim? A total of 10 changes an hour is not considered excessive. It seems that when many manufacturers state that a filter is for an x gallon tank, they're assuming a marginal five changes an hour. Perhaps another of the same filter would be adequate.

The 10x per hour is for HOBs, not for canisters. Canisters do not need that many turnovers to do the same of IMHO a better job.

I think you should get another canister. An Eheim or a Filstar would be a good choice. Runnning 2 canisters and having an inlet/outlet at each end of the tank will keep the gravel clean.

145 gph will only turn that tank over twice an hour, which is far too little. Even with a canister, a flow rate sufficient to turn over 5X the tank volume per hour would be the goal. Also, remember, these flow rates are under ideal conditions as posted by the manufacturer, not necessarily real world use (i.e filters become clogged, etc.) You need to find a way of putting together that flow rate, either with one canister large enough to do it, or two canisters if one isn't sufficient. As a cheaper solution you could also use that canister with a large HOB filter and improve filtration flow rates that way (it also gives you some back up filtration if one unit fails).

The delivery head height refers to the fact that as a canister is placed beneath the tank, allowing simple siphon of water into the canister, a pump is needed to push the water back up. That height rating is telling you that it can return water to a tank roughly 4 ft above the canister.

That would give you 440 gph which should turn over a 75g tank a little over 5X per hour. This could certainly work. Only one more thought to confuse things even more, but worth considering. canisters from 2 different manufacturers won't share parts and materials, whereas two identical canisters would allow you to just order one set of whatever you need. This is more an issue of convenience, and could always be addressed down the road.

There is absolutely no reason to turn over a tank 5x per hour to cover filtration by a good canister. None of my canister-filtered tanks need that. I do frequently add other units for more current, but not because they are required for filtration but because I like more current.

"delivery head m wat. col. 4 ft 3 in (what does this mean???)" = delivery head maximum water column 4ft 3in. Head is the difference in height between the water level (meniscus) in the tank and impeller/top of the canister. That traslates into the unit you have needing to be 4' or less below the water level of the tank.