I was in much the same position as you in my current project. I fully researched and decided on which projector I wanted before I began construction, but didn't want to actually have the thing on hand until it was ready to go on the ceiling.
There were a few reasons for this:
1) I was building in a new house under construction so I couldn't keep the projector on the job site.
2) I was buying a used CRT like you but with new tubes and I didn't want to waste warranty time sitting in a box.
3) These things weigh 150lbs+. I didn't want to have to move it around much.
Since I knew which projector, I was able to plan for screen size, throw distance, cables and mounting and such and I used unistrut to allow for fine adjustments after the fact.
As far as getting a deal, I ordered the unit a couple of months in advance of when I wanted it to be sure my supplier (Tim Martin, whom I highly recommend) could get me my projector exactly when I wanted it. Tim was great to work with and arranged to have the PJ arrive the day before I was to hang it.
This approach worked out really well for me.