HRE should certainly be your goal. Containment of France should probably be another. Austria can't form Germany without culture shifting. This would be difficult to do due to how many Austrian provinces you start with, plus ~10 stab to shift capital and culture. Probably isn't an option in MP, it normally isn't a good idea even in SP. Ignoring naval tech probably wouldn't hurt, as the emperor you will get auto-military access though all HRE provinces (note, you will have access, but not the increased support limit unless you get the actual treaty, which shouldn't be hard).
I would suggest getting as many personal unions as possible, which probably means spies which means mercantilism. Try to take over Venice for their COT. Genoa and Lubeck wouldn't hurt either. As for buildings, that depends. As the Emperor you will get a boost in manpower (I believe +1000 per imperial state, which, at the start, means +50000), which means that manpower may not be your biggest concern. Early game I would focus on production buildings, late game I would go level 5/6 trade buildings. They are ridiculously overpowered, and more so if you are a large nation. With manufactures, I would avoid building too many. They are helpful, but expensive. Buy them whenever their price decreases (based on manpower), if you decide that the money is better spent on them than troops.
