Just reading throught the posts and seeing if I can help!
From the two options, I would go and buy a new unit (from experience).
I suggest getting one that plays MP3's as you can put 250+ tracks on a single CD, all in nice folders/albums. (if you have a PC that its).
I've tried three types of FM transmitters and not one of them gave good sound what-so-ever along with being untidy and easily stolen if you have the window open in the wrong place at the wrong time
I ended up buying the transmitters and in the end buying the new head unit. So please, just skip the transmitter bit!
If you do still want to try it, you need to simply ask for an FM transmitter. Try Ebay to have a look at the sort of thing you get for your money.
A more expensive alternative would be to use these guys - Advanced Ice
or the like.
The bonus doing it this way means you get to keep your existing unit.
Hope this helps,