Help Newbie with a few questions

[Challenged_101]

Okay, I'm coming over from a flip phone and have a few questions. Just bought 2 MyT4Gs and a 3G since TMobile dropped the contract plans.

Downloading and installing apps...
I'd rather browse with my laptop, what is the process for downloading to the
laptop and installing to the phone. Does this very from one app store to
another?
Can I retrieve an app in the event I need to revert to a factory default?
After Googling the firmware, I found a link for updating the os to 4.2.1
Jellybean. What are some of the pros and cons.
I plan on following Phateless post on removing blotware. In the event that I
update to Jellybean, is it advised that I do the deletions before or after the
revision.

Thanks for getting me up and running.