Honestly, with the exception of the digital codes that help pinpoint more specific causes on one or two systems..... you can really get away with a home-brew blink code reader, which will pull codes from all systems (just not the few digital codes). A system like the Trisco PalmScan (which I have) makes all this just a BIT easier, because it provides all codes and readouts (again, sans the detailed digital codes) in a more user-frlendly handheld unit.
ONLY the MB HHT (handheld tester), the STAR C3/C4, and the SnapOn unit that Dave mentioned will give you live data. My experience, to be very honest, is that for the average Joe, me included, having live data is really something of a play-toy rather than something that is highly usable.
IMHO, playing with live data on the 036 is a form of automotive masturbation. (Psst, don't tell that to GSXR). The reason I say this is because as first-generation CAN + OBD1 cars, our 034/036 cars have very very limited windows into the systems, limited depth of diagnostic codes, and limited streams of live data. When you compare this to say, a W210 and then a 211 or 212, you see HUGE changes and additions as to what data is accessible, identifiable, diagnosable, and reprogrammable if needed. Live data can help in some complex automotive diagnostic situations, but how many times have you seen posts here to the effect of "I analyzed the live data stream coming from my 500E, and determined that XXXX was happening, which led me to believe that Condition YYYYY was present, resulting in fix ZZZZZ." Usually it's more of a visual/mechanical/blink code based process, at least on our cars.
Our 036 systems are very crude by modern standards, and even the C3/C4 only include the HHT to access the live data in an "emulation" mode, meaning the HHT is recreated in software, and combined with the rest of the C3/C4 which is designed for the more modern cars up to the present time.
So bottom line, and reinforcing my earlier point ... using the C3 is like using the fork on a Swiss Army knife to eat a piece of steak. Works great, but there are dozens of other tools that the knife has, that will never be used to eat that steak, yet could be useful for other types of cooking, starting fires, cutting, dressing a piece of game, and so on. While the C3 is the "ultimate" and most capable tool for our 036 diagnosis, we're only using, and only will ever use, a small fraction of its capabilities on the 036. That's why the blink code home-brewer, the Trisco, or the MB HHT hardware unit (which is as rare as hen's teeth) are attractive options, because they are much more designed to match our cars in terms of the capabilities they provide.
If I could ever find a used, but reasonably priced HHT, I'd snap it up in seconds, and probably sell my C3 to someone who can better use it than me. All I need is an HHT. I use my homebrewer MUCH more than I use my C3, BTW.
Cheers,
Gerry
P.S. It's all one reason why I love the 126. All you need to diagnose things there, is a home-brew blink code tester (and very few systems to test) and a $40 digital multimeter to test voltages in the fuel injection system, and duty cycle. For $50 worth of tools, you have everything you need. And the late 126 only has three computers ...
