nexus 4 t-mobile 4g question.

Ok so I'm planning on getting the nexus 4 through Google play and using it on the $30 dollar pre-paid plan. Here's a few questions:

1. Can I just walk up to a T-Mobile store and activate it there?

2. How do I use HSPA+? I mean like how do I turn it on, is there like a 4g toggle somewhere?

3. How fast is HSPA+? I'll be switching from a sprint wimax phone so is it faster than wimax?

Thanks for the help(: