Decide the focus of your collection before you start buying stuff up. Just certain characters? Vintage G1? Retro G1 (a la Generations / Reveal the Shield / Universe, etc)? Animated? Which characters or lines mean the most to you? Try to envision exactly what you want on your shelves. I grew up on TFs in the '80's, but got sucked back into it when my wife bought me a Premium movie Bumblebee back in '08 and I was in awe of the complexity, sophsitication, articulation, and artistry. Now I've got a six shelf movie collection depicting key scenes from all three movies that I love just staring at. Maybe once or twice a year I'll transform them or repose them.
If I were starting over, I'd stick with Masterpieces. Masterpieces look like the only line other than G1 that consistently goes up in value (if you're worried about resale). Hell, they're specifically made for collectors. If this is the route you're going, order MP Prime and Thundercracker from TRU while you can. If you have the money, snag Hot Rod while he still goes for around $100. That's as perfect a place as any to start since the MP line seems to have undergone an aesthetic reboot in the last two years. Even if you take them out of the box for display (which is the most satisfying part of collecting for me), keep all of the packaging. I think I'd be more fulfilled investing a few hundred a year on a couple top of the line iconic G1 figures than a few hundred a year on a slew of junk that no one will want or recognize in the near future, including you. But then, I grew up on G1, so G1 is Transformers to me.
One thing I have to say is that the TF community is amazing. I've met some awesome people on here who've been crucial to helping me complete my collection.