If I may add my .02...
If you are going to a 250... I wouldn't go with a single return for one reason... redundancy. Protect your investment. I currently have a large single external return pump that I will be converting to 2 external returns over the winter for this reason. Something I should have done from the start.
You are about to make a significant investment in "stuff"... most importantly livestock. What do you do when the pump fails? Or you need to replace an impeller? Or something else... You have then isolated the two water volumes (tank from Sump) with no turnover while you wait for a fix or can locate a spare pump. I would design 2 returns if for no other reason than peace of mind. You certainly have the sump space to support it.
If internal like a Mag...What do you plan for lighting? The reason I ask... you may want to consider heat. If you plan to use heat intense lighting like MH, then heat may be a consideration for you because cooling a large water volume is potentially expensive. If you run MH you will be radiating heat, add to that an internal pump (or pumps) that are increasing your tank temp as they cool. You may come to a point where in a large water volume controlling or reducing your temp will be more expensive than what you might spend using external pumps. You could get away with cost effective fans to increase evap, or have to go the expensive chiller route. That's a choice/risk you need to decide on but should be considered when building an entire system.
For that reason I prefer external pumps but you may not have room under the stand so internal may be your only solution. There are a lot of suggestions on these forums for quiet external pumps that can be plumbed into your sump.
Speaking of, the way you described the water flow (2 Drains -> Fuge -> return) is putting 100% of your tank volume through the fuge. It certainly works, but some opinions believe fuge flow should be slower, certainly not the same turnover rate as your tank. Depending on the opinions you may see something like 10X turnover for the drains, 3X or slower for the fuge per hour. The reason is the slower flow gives slower, flow averse critters a happier life, time to grow, etc. This also lets you light 1 side of your sump instead of the middle which means you can run the other sides dark. This reduces algae and future maint. as a result that we all grow lazy with over time. Again, this is a decision you have to make but if it were me, I would split a drain line and feed the fuge with that leaving your return chamber in the middle. (2 Drains -> Return chamber <- fuge, T from 1 drain).
enjoy and take your time with it 
