- I need to play Horizons on Steam instead of using the original game?
No. You can, if you generate a Steam Key through Frontier's store (3rd party keys or so), but you don't have to.
- But I do log into my Elite Account anyway so why?
They offer steam keys for convenience. They have to pay shares of your future purchases, skins, bobbleheads etc., on their store to Valve, if you generated a key however.
- Can I uninstall the original client?
Yes. It's a folder in the installation directory, next to Horizons.
- Can I still play with my girlfriend who only has the base game (so far)?
Lucky you! Yes, you can. She will play on the Elite client (32 or 64 bit) and you will play on the Horizons client (64 bit only). This is "1.5" as vanilla update and "2.0" as Horizons update respectively. The players will see each other, but vanilla players will obviously not be able to approach planets as well as use other 2.0 exclusive features.
- How? With Horizons Client? With both?
See above.
Edit: beaten. ^^