GPS Speedometers
 

Looking for some advice!
Looking at buying a new GPS Speedo and also a new GPS System.
I wanted to get a new GPS unit with reciever.
I also wanted to buy a new GPS built in dash speedometer.
BUT I DID NOT WANT TO BUY ANOTHER RECIVER JUST FOR THE SPEEDO!
Reading some of the threads, there seems to be some comments regarding the speedo's not working accurately plugged into the gps systems without a transformer in line.
There are so many choices, Auto Meter, Gaffrig,Harden Marine Livorsi,Nordskog, who makes the best unit?
Garmin, Magellin, etc etc etc.
Any advice would be appreciated!