I have been poking at DSM's for 10 years now and I am still learning new things. I made my PDA logger cable out of old TV parts =\
Grats on the DSMlink purchase, I will be going this route once I can justify the cost. I will be building a cigar box computer hooked to my navigation which will be hooked to the DSMlink with some very compact wireless keyboard and mouse. This will also serve as a mp3 storage unit for muh tunz.
However, these guy have a very valid point. Most of what I have learned was by trial, error and lots of money. That and the endless information on DSM sites. I think the point everyone is trying to make is the 495$ spent on your last purchase could have gone to new seals, wiring and other trouble shooting / problems. While I don't know your financial status and skill level let alone your car setup, your mechanic idea is a step in the right direction.
I hope things work out for you and you enjoy the GVR4 experience.