1) Use this link for space. For Ground, there really isn't too much to it, but if it helps: Splitbeam rifles are ideal, Fleet armour with CrtD, and the Na'kuhl Temporal Operative shield are the safe bets. Anything else would really be your own trial and error testing to what you want.

2) Yes.

3) Honestly, the only thing you need to worry about when picking a side is if you want to play with other people in, say, missions. Otherwise there really isn't anything to dictate one choice being better than the other.

4) The expansion packs you see on Steam are mostly for ships. So, unless you plan to use all of the ships in a pack (which you won't anyway with the Romulan one), they're not overly worth it. That said, the Agents of Yesterday pack is definitely the best bang-for-your-buck out of the three, since it offers 7 T6 ships on top of one T5, T4, T3, T2, and T1 ship, and other trinkets. The Steam starter pack is really just for those who love the Steamrunner itself.

5) Buying the lifetime subscription - as opposed to the monthly - offers continued access to an extra 1000 Dilithium refinement every 48 hours, alongside a 500 Zen stipend every month. Unless you plan to play the game for years to come (which is too early to be making that kind of decision, really), you should just stick with the monthly which offers, what, 600% of its value in one convenient package.