I have a firewire enclosure and as far as my experience goes, they are pretty easy to install yourself.
Installing a drive inside it is about the same as installing the drive inside a computer, so if you done that before than don't worry about it
It has an flat cable connection, and a power supply just like the inside of a desktop computer, simply attach those as you would do with the computer (you can't go wrong actually as the flatcable (IDE i guess) and the power connector can only be attach in one way).
Close the encloser up, connect to power and firewire and it's all ready to go.
Not sure if you are interested, but a good thing might be to search for ones that hold either a harddrive OR a CD-R (CD-RW, DVD-Rom, DVD-R... etc) in their enclosere.
I can build either one i like in mine (for the moment it holds my old DVD-R that came out my fried Quicksilver).
The good thing about firewire is, that most enclosers (i think) come with double ports, so when your firewire drive is in use, and you are in need of another firewire device you can just link it on the drive and create a chain.