A mag-drive 18 is not going to push that..... The 20' of horizontal isn't technically "head pressure", but it is additional resistance. Add that in with the 20' vertical, plus all the elbows, and you're going to need a serious
EDIT: Here's a Mag-drive Supreme 18 at Marine Depot, and down the page in the "Product Description" tab, it shows the head pressure spec sheet for their pumps. 20 straight
feet of head pressure is the shut down for the 18, not including all your horizontal & elbow resistances.... and elbows add serious resistance. Whatever you use, you're better off going with dual 45's rather than a single 90 wherever you can.
Danner Mag-Drive Supreme Water Pump
EDIT 2: That kind of run and height may give you some problems..... Even the ReeFlo Hammerhead at BRS has a MAX head of 23'. The Manta Ray or Tiger Shark looks like they could handle it, but then you're getting into some serious power consumption. The Tiger Shark is 1065W.... That's like leaving a hair dryer running 24/7.