According to the R1 trailer, the Empire seems to have gone in the reverse order when building the first Death Star (as compared to the second one in ROTJ):
- DS1: the entire battle station is actually built first, and then the Superlaser focusing dish is placed at the completion stage.
- DS2: the Superlaser dish and the surrounding area are built first, the rest remains +/- unfinished.
Interestingly, the finale of ROTS (though I personally prefer not regard the PT as canon) shows a DS1 superstructure with the superlaser dish skeleton already in place.
In the old EU, that was a test platform, not the first Death Star. Even down to the same skeletal design. I’d say it is more likely to be that and they spent quite a few years perfecting its systems before they started building Death Star I. It is also possible that Death Star II was being designed already (this has happened many many times in real military development).