The Blueline 40 is a lot of pump for a 40 breeder and a 20G sump. (I was using the 40 HD-x on a 180G with a 40G sump!
While your overflow may have 800 gph capacity, the question is do you really want that much flow through your sump. Will there be a skimmer in the sump? Other filtration?
If you have your heart set on a Blueline, you can always throttle it back the outflow side (only) and you'll be able to use it as you upgrade. It's very relaible, quiet and a nice unit. I still use mine in a water change system.
For your size tank, you might want to look at the Eheim - more compact, quieter and a little less pricey.
I think it just depends it fou're going to be upgrading anytime soon...
JMHO
LL