No need to buy the unit unless you are all out DIY'er.However as I just posted on an OBD2 thread.this site has tons of great info on engine diagnostics,codes,and how to track down what the code is potentially trying to tell you,all FREE.
A great read,some night when you have nothing to do and want to learn up on how systems operate.
ohlord AutoTap - OBDII Diagnostic Scanner
join their email list and they sent out a great news letter full of info......
I just ordered my new Magnecor spark plug wires 2 days ago (8.5mm Magnecor part #85240 for E55) for really cheap! I mean I researched the crap out of these and they're going for $251 bucks plus shipping everywhere including Magnecor's website. I found a site called Alamo Motorsports where I ordered them for $170.02 shipped. Here is the site's link to the wires: Magnecor Spark Plug Wire For Mercedes
Free EPC for USA cars. No need to go to some Russian site that you may not trust.
Go to http://epc.startekinfo.com and hit the "Subscribe" button. The credit card info is still marked with an asterisk as required and I did enter one, but the price is listed as "FREE" for the USA cars. It's $12/year for world cars.
Note: if you have a HFT store near you, print out the page with your item on it or the physical store won't meet that price. Odd rule but it works fine once you know it.
Take care and enjoy the ride,
Greg
__________________ When you learn from your own mistakes, that's experience.
When you learn from the mistakes of others, that's wisdom.
When you fail to learn from any mistakes, that's government.
RACK! 
OBDII code readers 
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