The last few releases of Mac OS X really push of disk I/0, something SSDs are dramatically better at that HDDs. Using an SSD, either internally or externally, makes your system run better than new. Internal is better, though.
Helpful hint:
use a cardboard the same size of the computer, 3 ply, draw the computer on the board and place the many different screws exactly where they they are on the motherboard. Circle or put a number clockwise after taking out a screw. This makes reassembly easier without meassuring the screws or looking at Ifixit pages. And the screws will not move about or get lost.
You will thank me later!
I swapped the HDD for an SSD. I did not find it difficult at all. I do recommend that you have the proper tools, a clean workspace, and the iFixit guide next to you.
I did not even remember that there were security screws, so I suppose the iFixit tool kit just already includes security bits.
If you are uncomfortable working on things, you could always do an external drive like others have mentioned. This avoids the risk of breaking fragile connectors, which some people here have done.
Personally, I strongly prefer internal. There are many benefits to internal and I've never broken anything inside a Mini doing upgrades.
I have upgraded many model years of Mac Mini and I think the hardest was the first because that required a putty knife.