You buy tech and military at the same time. How you split up your resources is based on what type of strategy you're playing. If you're all exploring, then you just need turrets, if you're attacking then jets and turrets. The important thing with military is to try and buy enough so no one can effectively land grab you, if people at your networth have 500k jets and you only have 200k turrets, you're going to get attacked multiple times a day, but if you have a million, then people will avoid you. So you have to constantly buy military as the set goes on. Keep in mind that attackers have to buy defense and offense, so theoretically you should be able to buy more defense than they have attacking units, at least at the start of the set when everyone is exploring. A good rule of thumb is to try and maintain a high NW/land ratio. Defensive allies are also very important if you are all exploring.

You're right about teching, a lot of people start by teching, then switch their buildings over to the main strat, especially as tech prices are high in the beginning and drop as the set progresses. Sometimes it can be more effective, but its a little harder to pull off because non-teching countries can generally grow faster than you at the start (especially commies), and in an individual server you might get attacked. This does make your country more efficient then others around you, so you can have less land and similar production, which makes you less of a target as well.

I think this thread should also contain links to eestats (http://www.eestats.com/) and earthgraphs (http://www.earthgraphs.com/). they really are useful resources to have and new players usually have no idea they exist.