Generally speaking, with regards to warranties, if your installation bodges up the rest of the computer, then you will not be covered. That said, the instructions provided for internal drives can be quite good (I remember old HP drives used to even come with little video's on the CD to show you how to install it all!).
However, performance of external drives have been getting better over the years.
What it really comes down to is if you are worried of getting inside your computer - you will be paying over the odds for an external drive, but you won't have the inconvenience of taking the lid off your PC.
As to what you actually need to DO to install a CDRW drive, then click here to view an example - if that looks daunting, then get an external one!
Hope this helps