This is depends on way you want to build your army: sarurus, skinks or big monsters? And if you want to play only with friends or participate in tournaments?
Start Collecting is always a good choice, especially if you want a Saurus army. Besides, you can easily make several good conversions like Scar-Veteran on Cold One and Astrolith Bearer. From the other hand, Saurus army will take time to build. You must have at least 40 warrors and 20 knights for your lists to work properly. So you'll need 2-3 SC boxes and 1-2 Warriors box. For larger games just get more Warrors (60-100 models total is OK) and Saurus Heroes - Sunblood and Oldblood. You can build towards Sunclaw Starhost, Fangs of Sotek or Dracotions Tail, if you go saurus heavy. All of these battalions can be really strong if you know how to use them.
If you want large monsters or skinks, a Skink Starpriest, a box of Skinks and Bastiladon could be a better choice. Separately, it will cost some more than SC box, but you will have an HQ, 2 battleline choices and a Behemoth - this will be 480 pts total, about the same amount of SC box. You will definitely want want these models when you shift to larger games. Alternatively, if you don't want big monsters, take a box of Terradon Riders instead of Bastiladon. From this point, you will need just buy more skinks + big monsters/fliers. Y
In either case, get slann ASAP, He is mandatory for 1000+ pts games.