Upgrade or Return?

I recently purchased a 2013 21.5" iMac from Best Buy, base model, and am pleased with the performance of it. However, going from a MBA with a SSD to iMac has been an adjustment. I've been thinking about returning my iMac for a BTO one from Apple that has the SSD, here are a few simple questions I need some advice/help with!

1. If going the BTO route with Apple, should I get the 256gb SSD or 512? If I decide to get the 256gb SSD, can I later replace it with a higher capacity SSD or would it be better to order the higher capacity to begin with? Money is a strong consideration, would like to get the 256 and later upgrade it myself if that is possible.
2. Would a better option be to buy a 480gb SSD, hook it up via a mSata USB connector, and use that as my startup disk to run my computer off of? Instead of running my current computer on the HD, I would hook the SSD up and run my computer off of that.

Thanks in advance for the help, I will be graduating college soon and I want to get the best option now for many future years of use.

 

Anyone have some advice or recommendation on what would be better?

 

What about getting a SATA to USB adapter and hooking up an external drive to it, would that be a good options? I've had a hard time finding one of those or an external SSD thats 480/512gb anywhere.

 

Alright thanks for the help everyone, I'm still debating what to do but have a little time to research some more and think about things.