Your first purchase should be the Army Book and the Battalion. The Lizardmen Battalion is probably the best out of all the ones available right now. After that, you should probably get a Stegadon. With some magnetizing skills, you can make it so it can be any of the three options. Next, you should probably get at least another box each of Saurus Warriors and Skinks. With all the extra bits, you can probably kitbash a couple Saurus Heroes for your armies (the TG sprues have multiple extra heads, the Cold One Riders are also laden with extra bits and bobs). Once you have a good foundation of plastic minis, you can start getting the fun metal stuff, like a Slann Mage Priest, some Kroxigors, and some Salamanders.
I got 2 battalion boxes, a slann and a stegadon. With some really simple conversions, I'm building some saurus heroes and some chameleon skinks. I can field a nice 2500 force. My most important advice to you is to pick the models that you like. In the end, you aren't only playing, you are also painting those miniatures. To me, looks is as important as effectiveness.