A sump is a good idea, and make it a fuge by adding some terrestrial plants (like spath) to filter out some nitrates, not a bad idea.
For such a large tank (assuming 300-350 gallons and not liters) I'd go with a large capacity Mag-Drive or similar pump, something that can crank out around 700-800gph at 4 feet of head. If you get one even bigger, you can have multiple outlets for seperate "trickles" and such. If you have a really nice flow rate, you can still keep slow-moving fish by using deep "ponds" and plenty of rock/driftwood in the lower pool.
Have the sump under the tank, and I'd use a ton of mechanical filtration since you may have a problem with dirt from any planting in the land part of the tank. Make the sump as big (in water capacity) as you can stand, dilution is the solution.