I'd recommend starting with the Battleforce, as it's great value for money and gives you a neat core force to work with. Tau are, arguably, characterized by their railguns so it wouldn't be remiss to add a few broadsides or hammerheads next.
You'll also want more crisis suits as they lend massive versatility to you force. On the note of crisis suits, if you're a good modeler, you might want to try magnetizing the hard points. That way you can re-customize each suit as needed, instead of spending trucks of money on dozens of differently configured suits.
Maybe you could give us an idea of how you used to run your old force and what style of play you're looking for?
I used to use Fire Warriors dismounted to deny area to enemy infantry, with railgun heavy support and crisis suits to do the real killing of the army. Then, when the enemy was weak enough, I'd run the FWs up to the enemy objectives in 'fish and finish them off.
I sometimes used Kroot as cheap and disposable shooters against swarm armies in a sort of Roman style checker formation, but I'm not sure how effective that would be in 5th. I suppose it would depend on terrain. However, I'll have plenty of Kroot with the battleforce(s), so I can choose whether or not to use them regardless.
So we're looking at mobile infantry, suit heavy and railgun heavy.
That should give me plenty of options.
Fluff for this army, in case anyone is interested:http://www.40konline.com/index.php?topic=202985.0
Edit: Modify, not multiply! - oink