I highly recommend those Newertech units. I use one with each of my Mini's and they are great!
If you install a fast 3.5" drive your Mini will actually perform better if you install OS X on it and run it form there instead of the internal Mac Mini drive.
As far as the OP - Taking apart the Mini is a b!tch, even if you don't care about it. You'll need to fashion a pry tool from a medium size putty knife (it needs to be sanded to more of an angled point). Once you do, popping open the Mini is not overly difficult, just be careful of your fingers. Now that you have the case off, there are several screws that need to be removed.
I mean I'd do it in a heart beat but it sounds like you're not too sure. I don't think it would be worth it to pay someone to pull out your used hard drive. Take the same amount of money, invest in a nice large (up to 750GB!) 3.5" HD and pick up a Ministack.
🙂
Greg
Edit -
Ah, just noticed you'd like to get your old files off! Send me your Mini, I'll pull out your hard drive, install it in a 2.5" enclosure for
$10.00
🙂