Thats right then you would need an LNB, which would take the place of an LNA. All an LNB really is, is a preamp that downconverts at the same time. Many that have the capability to listen at 1.5 still downconvert because of coax loss issues, so its not uncommon. Down east should still be able to help you out.
All that stuff sounds great, you are almost there. One of the old dishes, coax and connecters will work great. You will still need to build a feed for the 1.55 gig range which you should have no problem with, just search for inmarsat feed. I dont think you will be able to get a preamp out of that stuff, but they are not too expensive, and not needed right off the bat, but once you run coax any reasonable length you will need it. Without it the signal will be near or in the noise. Give it a try its a lot of fun!