There's no such thing as "unlimited data" for mobile broadband. They all have fair usage caps and they vary by providers, some as low as 250mb/mo to 5gb/mo. I use XL on my unlocked iPhone and it's been working fine for me, I also have a company blackberry and they use Telkomsel.
Best carrier would depend on where you plan on using your phone, personally, I think telkomsel has the best signal quality through out Indonesia. XL is a bit hit and miss but Jakarta area is pretty reliable.
You can pick your own phone number, they're listed on the pre-paid card cover when you buy them. Pre-paid sim cards are available everywhere, even street vendors sell them, so you won't have trouble getting them. Instead of going to Electronic City, I'd recommend Ambasador Mall in Kuningan (near SCBD) to find your sim card, you'll find more choices there and usually the vendors can help you activate the sim card, help you set up and maybe even advise you of which plans to use.