In the states it is becoming more commonplace because more manufacturers are incorporating the factory systems into cars with the headunit directly interfaced with AC and security systems of the vehicle. As such, the unit is basically a closed loop unless access is gained through the changer interface as these other devices do.
To integrate anything into the closed loop without knowing the exact specifics of the factory radio would be overly time consuming and ultimately only applicable to the one stereo and imagine the mutlitudes of different OEM stereos in cars. Its just a nightmare to be honest. The amount of work to be done to integrate one portable device into the car after research, parts finding and final installation isn't worth the time in the end, esp. in NZ where target market for aftermarket in car audio equipment is low.
Retailers would be keen to push a newer unit on you to yes, make a sale, but to dcerease the amount of work that is required to get you up and running with an interfaced device.
So your options are then the interface that you have found and install yourself, get a modulator or to get one of the devices i pointed towards previously.
Personally if you are on a budget then i'd get a modulator. Saves having to import the interface and waiting for shipping time. Also, you wouldn't have to open up the dash (doesn't sound like you are confident in doing that) and would save you time.
If you were truly into sound quality then you would have already binned the factory unit in favour for a better unit and better equipment in general. The realisation that you get out what you get in applys greatly to MP3 devices, in the fact that unless your music is encoded to 192kbps VBR and above, you really aren't going to notice a great difference between lower kbps encoded MP3s and compressed radio.
Go the FM modulator. fastest and easiest route unless you want the specific interface and are willing to take the time to do the work yourself and find out which wire does which and what the change pin config is (i read in a foreign language that is pins 18/19/20?
link
choice is yours.