First off welcome to 40k online
Building on the army you already have, get some crisis battlesuits and a squad of firewarriors. Those models will give you a solid force for small games (like 40k in 40mins), and will help you get an idea where you want to go next. A devilfish troop carrier would not go amiss either, as it can help build the mobility that many new tau players lack. Then again, you might want to get more models before you go to vehicles. Either way it is your call.
Points wise... hmm... assuming that your kroot box gives you 16 models, and your fws come with drones, then the force you have is about 300 pts. I am not sure what the exact total is, but it is around that number. If you want to find out exactly how much, then review the codex and familiarise yourself with it.
Hope you enjoy playing with your Tau