If you don't like the way you look with your current set, but still have an older one that you liked, you can always drag another set into your "costume" slots. That'll make you appear as if you're wearing the other armor, while still having the effect of the set you currently have. I, personally, like to collect armor, so I have a few different sets even though I only use one or two. It's not uncommon to have more than one set.
Autogun? I have the same problem. I miss the run and gun style of other guns, like the alchemers. Currently, though, I don't have the resources to get a new one that is worth it. So I'm sticking it out until I DO get the resources.
Whether or not you should upgrade or change your equipment for 3*, well, that's up to you. Do you like the stats for your current armor (and it's 3* upgrade), or would you prefer the stats of another armor?
If it's purely for aesthetic reasons, then I say maybe save up a little at a time for a set that you prefer to be seen wearing. Or you can put off upgrading to better armor until you get the costume armor you want. You can see what other armor looks like here.

As far as the energy question, yes, you CAN share your energy! Crystal Energy (CE) is what you buy and it can be traded. If you and your friend want to pool your money together to buy energy and then split it, you can! :D
I have bought energy, and personally I thought it was worth it.

Hope this helps.
~O