Any upgrade in spring rate front and rear will do wonders for our rigs. I'm using the toytec 620lb front spring at the OME 895 in the rear.
If you can afford to I wouldn't bother with used coils from anywhere. But if they are free and temporary, I say go for it. Yet if you are trying to build a system once and once only, then yes upgrade the coils front and rear. Also a good rebuildable shock with a reservoir would be the hot ticket. I'm ultimately going with the new Fox Dsc's pretty soon. Their package comes with a new coil up front, and it's supposed to be valved for our rigs out of the box. In the rear, I'm going with a new multi rate coil and the Fox Dsc's.
If I could rebuild my Bilstein's, I might keep them. The rebound in the rear is a bit off for my tastes, but again for the low cost of my system, it's really hard to complain.