Since it is quite old I'm not sure what is readily available that was when it first came out. I'd definitely get an AI program because the default AI sucks. I used skAI for a long time, but there are quite a few out there. IMO AI is a must and most decent programs have real-world liveries with real routes, once you spawn in and whatnot you might face actual traffic congestion at JFK, LAX and other large airports.
It really depends if you want to put some (more) money into the game. There are a lot of payware addons that make the game look gorgeous. I'd check out simviation so you can download some free stuff such as airport graphics, maybe some lighting stuff, different planes and liveries. Some payware addons have multiple airport graphical updates as well as general area scenery (NYC, LA) that the base version of FSX does not have.
I would hope that the steam version allows you to install addons like normal.
