My incomplete list:
Sensors capable of detecting a populated planet from pretty far out. This can mean large sensors, or it can mean long ranged sensor probe missiles. Which means a ship to launch them, and all the missile technology that means. Of course, you don't need a very big missile to detect a civilization, once it gets close.
Getting your missile shot down counts as a successful 'detection'.
My planets must have SOME missile defense. Maybe meson PDCs if I had a good EW scientist, but more likely it will be PDCs with AMM launchers. And again, I feel I need anti-missile sensor systems developed before I probe another system.
I figure I absolutely have to have a jump drive capable of getting my survey ships through, and possibly my small freighter/colony ship, the ones under 12k or so. I definitely want a tanker of some kind, to set up a forward fuel base, although I generally survey with such efficient drives that my survey forces will complete the survey before they run low on fuel, at least for small to average systems within a jump of my home system.
I also want the capability of building a fleet in a hurry. And while I really like missiles, it is kind of difficult to build up both ships and missiles at the same time. If I am building a fleet in a hurry, I would be doing things like mass producing lasers, power plants, and engines with planetary industry, in order to turn out ships as fast as possible.
The next issue has to do with economic exploitation. What distance do you exploit at what engine tech level? Do you build jump gates so your civilians can get in on the exploitation, or just build a civilian jump ship to escort convoys?
If you started with missile bases, do you build real missile fire controls for them, and build some size 24 missiles to shoot from them, or do you go with smaller, easier to research missiles?
What kind of shortages prompt you to head out early? What kind of resources would prompt you to significantly delay?
For example, I am contemplating a serious mercassium shortage. Venus has lots at .2, along with duranium at .6 and some other stuff, so it is economically mineable, sort of. But there isn't another deposit larger than 10k in the solar system. Teching up in the solar system isn't possible without more mercassium, or without building a crap ton of automated mines for a .2 accessibility resource.