Mac Minis are great machines for folks who don’t want to break the bank but can’t really stand anything but a Mac. They don’t take a whole lot of space and are easy to repair/upgrade. You are going to need these Mac Mini repair/upgrade kits though:
iFixit Mac Mini Dual Hard Drive Kit: helps you upgrade your Mac. It has the tools you need to add a second hard drive to your Mac mini. PDF instructions are also available.
Crucial 16GB Kit: Apple does charge a considerable amount of money for Mac upgrades. You could always do it on your own though. You just need this kit to get started.
Silverhill 20 Piece Tool Kit for Apple Products: many of you have these tools already. But if you are starting from scratch, you should consider giving this toolkit a look.
These kits help you upgrade or repair your Mac Mini. If you need help, sites like iFixIt have plenty of guides for you.