Changing things up on my desk. MacBook Pro wants to take over!

Hey Guys!

So I have this kind of complicated problem and I would really like to get some advise from you! :)

Let's get right into it: Currently I have a 27" Late 2009 iMac with 1TB HDD, 8GB RAM and a Core 2 Duo Processor. I also happen to own a 2012 Retina MacBook Pro. That has 128GB SSD, 8GB RAM and the standart Processor.

Now I have been thinking a lot lately and I came to the conclusion that I should maybe sell my iMac. I need my MacBook for School/Work so I need to be flexible.

When I see my iMac and get around 700 Euros (I live in Germany) for it I would need to get a external HDD and a Display from that money.

1. -> Do I also need one of those fancy Docks out there selling for over 100 Euros? Is their a cheaper way?

2. -> What kind of Display would get close to that iMac Display? Sizewise it would be from 23"-27" but I really don't know what kind of display to get quality wise. If I choose a 23" model a Full HD resolution would probably be fine but what when I get a 27"? And also: Quality? What kind of Display should I get which has the best quality for the least amount of money?

3. Right now I save pretty much everything on my 1TB HDD in my iMac. I also do Backups on my 1TB external drive. If I do this, I would want to buy a 2TB external Drive with USB 3.0 so I can use that to put all my files and stuff. Would that work? Also -> I use iTunes and my iTunes Library is quite big. When I have my Library on the external Drive and have that plugged in, but then plug it out (going to school for example) will be library "break" and do I have to import it again every time or can iTunes just not "play" anything? What would you do about the Space problem?

Thanks a lot!