Free Trial -> Real Game

[Erendis]

If you buy it on steam, you will have to buy expansion there too. And I do not think that you can unbind from steam.

[Vandril]

If you buy it on steam, you will have to buy expansion there too. And I do not think that you can unbind from steam.

This. Aside from that, it's largely the same as the PC version.

Even if you buy the PC version, you can still use the Steam overlay by running the launcher through Steam. I do it. The only thing you don't get is easy access to the FFXIV Steam community through a link from the game listing - you'll have to go there manually. There might also be restrictions in how you interact with the FFXIV Steam community if you don't buy the Steam version of the game, but I'm not sure.

[Niwashi]

2. Can I buy the main-game first and level to that max - was it 50? - and then go buy the expac and continue?

The only benefit the expansion gives below level 50 is the ability to make an Au Ra character. If you like that race and want to switch to it, get the expansion early. The only other reason for getting the expansion now would be if you want to buy the bundled version of game+expansion.

Should I, or should I not, buy the game/exp from Steam?

If your trial is through Steam, then get the game from them as well. If it's not, then don't. You won't be able to have both Steam and non-Steam PC codes apply to the same account. (Though console codes can be combined with either.)