Many ways to do, but my suggestion would be to combine a small sump pump with one or more small power heads internal to the main tank. Typically if you do not have to rely on the sump pump for all your circulation needs you can get by with a pump that turns over you tank only 2-3 times an hour to allow proper temperature control (assumes heater in sump) and to supply any additional filters operating from the sump. For your 40 gal tank that means about 120 gal/hr, allowing for a head loss of say 100% (will highly depend on pump type but this should be sufficient for most pumps with at least a 8-10' max head or so) you need about 240 gal/hr or more. Adding some additional flow rate capability that you can always valve down if needed, I would round this up to say a 300-350 gal/hr sump pump (external or submersible).
You can then add one or more (will depend on flow requirements of planned tank inhabitants) smaller power heads inside of the tank of say 150-200 gal/hr capability each and get the advantage of more flexibility in setting up/adjusting circulation flow patterns in the tank (especially should you decide to add wavemaker) as well as the added reliability that multiple pumps bring to your system. About the only equipment failure likely to adversely effect the tank quicker than no circulation in tank due to pump failure is a failure of the tank itself.
A bit more complex than you may have had in mind but I think more likely to help with your long term tank success than a single sump pump would.