My first advice - don't get too caught up on features or titles. There are a ton of good games. A lot of titles have come out of nowhere as darlings. That's often because it's featured in a tournament lineup, or a member of the community anoints it as something special. Those games were good before they were well-revered, and it's fun to discover them on your own! Also, if you have space for 2 games, and get 2 of your grails right away, you have less flexibility when it comes to picking up new machines later (which you'll likely want to do).
That said, I think early Williams games are the best value right now. The best Bally games are highly sought after.. I'd just avoid that arena for now until you decide what you really like. Early Sterns are the latest to see a big bump in price, but there are some really good titles that haven't seen a huge increase so far (Meteor being among my favorite of those). Gottlieb has some good titles in that era, but I find them more difficult to make reliable.
The era you mention is my favorite. The titles with speech really start to limit you, though. I've owned Gorgar. What a gorgeous game.. I loved the way it looked. The gameplay, on the other hand, gets rather dull.
Really, I'd suggest picking up a solid working game of the era and just playing the shit out of it for a while. You'll get your flipper skills on practically any game. As long as you don't overpay, you can typically get a good amount of your money out of it.