Before deciding to buy MB STAR C3, sdconnect c4 / SD C5, you need to answer a couple questions why you want a MUX/Diagnostics.
1. Do you just want to read and reset fault codesin your car?
2. Do you want to change configurations?
3. Do you want to purchase something now that you can use for any newer cars you may get?
If you only want to read and reset fault codes you do not need ANY MUX (C3/C4) at all!! You can just use a passthru cable for around $20 or get fancy and get a wireless passthru “cable”. There are limitations with what a passthru cable can do but not sure what. I only used it to read initial codes on a 2017 GLA 250 while I was waiting for my C4 to arrive. I used a Toyota MVCI Xhorse clone.
If you want more support capability than a passthru you only need a C3 MUX for your car and use XENTRY/DAS 11/2012 version (this is the last most thorough version compatible with C3 MUX)If you want to purchase a “future proof” mux that can read old and new cars then purchase a C4 MUX and use the latest XENTRY 2019 version. C3 MUX with 2012 DAS software can work up to 2011 W204 from my own experience.
There are different chipsets (better or cheaper clone copies) on the circuit boards in the C4 MUX so you have to be careful which you buy — many places may have just as good of quality mux but this is where I purchased mine with the better chips (verified once I received it), good packaging, fast shipping and communicated well prior to buying it
Recently, there is one sdconnect C4 Plus MUX, which is needed for some newer cars with DoIP protocol like W222, covering Benz cars from 1989 to 2019 year.
What make it difference is it is compatible with DTS Monaco and Vediamo.
C5 MUXs that are being sold are mostly FAKE — they are only C4 MUX with upgraded housings. You can mod a C4 for DoIP if needed. A true C5 is very expensive and has its own HDD.
You can get the software from cartechnology.co.uk for free and another great site is mhhauto.com but have to pay $25 to register — both sites are invaluable!! I have had to use information from both sites.
I myself went overboard and spent a bit on a Samsung 9 Pro laptop upgraded to a 1TB NvME SSD drive with Windows 10 that converts into a tablet that I use dedicated to my car diagnostic programs (325GB+ just in XENTRY/Vediamo/EPC/WIS/Starfinder/SDMedia) I use this wirelessly with a C4 MUX.
If you are not computer literate then by all means purchase a complete set with MUX and laptop with software preloaded. Anyhow hope this helps get you in the right direction but most users can get by with everything needed for only a few dollars using just a passthru cable.
Attach one image of MUX and laptop with software preloaded: