After being gone for such a period of time, and Engineer being one of the more difficult professions to play, you'd be better off starting fresh either way. It would likely take more time to figure out what all of your skills, nanos, and attacks do again and how to use them properly and effectively, than to learn them by starting over. It would give you a better sense of what you can do (and consequently, can't do) instead of relying on memories you may or may not have from before. AO definitely has one of the more complex skill systems, and with a trade skill profession like an Engineer, that can be doubly as complex.
As I understand it now, there isn't an option to upgrade per expansion, but only as a package with all of them. There was an option before to do only Shadowlands, but I believe that has been discontinued and it is no longer split. You would get Shadowlands, Alien Invasion, Lost Eden, and Legacy of the Xan all together. Which if you're planning to go the subscription route anyway, is the best since you will have SL and AI Perks, and research from LE that all make a major difference in what you can do.
You may be required to reactivate the account by email, as they have suspended inactive accounts, likely due to help with the server merge that's soon to be coming. Once that happens you'll see special deals at the in-game store that are available and can include the expansions, game time, and other extra goodies. Those are generally for the more long term, but the option to just get the expansions, then pay monthly is still there as well.
Hope that helps! Feel free to ask anything else you may be wondering about
