Alex
For Era III there is a good selection if you are also willing to consider the possibility of non-Märklin, but restricted by your overall length;
Already mentioned is the Uerdingen railbus (Branch Line Growlers and Märklin have made many versions (including Austrian blue and white) including snow versions and weather versions and numerous advertising versions. There are also some specials editions and the option to make it three cars long. Märklin also made the older version which has been subject to a number of special livery resprays and they can be digitized for something different on your railway. There are also older versions made by Primex.
Marklin did the BR 515 battery railcar which usually runs with two cars (too long) but can be run as a single car. There is a newer digital version available at good prices but plenty of older ones available which can be digitized.
Marklin did the Kittel railcars in various livery as you have mentioned.
Marklin have made the VT 859 railcar in various livery, but all will need upgrading to digital
Marklin have made the VT 75.9 + VB 140 trailer, these are available with our without sound in digital.
Marklin have made to VT 135 Schienen-Omnibus Wismar in various liveries, I think all would need digital upgrade.
Marklin have made the class eIT 1999 "Glass Train" electric in various liveries
Marklin have also made various SVT 04.5 railcars in various liveries but slightly too long for your stated length but look great on a layout
Away from Marklin many suppliers have made the same or similar items to Marklin listed above, but have also offered something different, and there are some nicely detailed railcars (some may need upgrade to 3 rail or digital operation);
Brawa have made VT137 'Stettin' railcar in various liveries they do two car version which is too long for your stated length but also single car versions
Brawa have made ET 89 "Rübezahl" in various liveries
Brawa have made VT 761 in various liveries
Brawa also made the East German version of the Uerdingen railbus BR 172
Roco have made ET 90 in various liveries
Liliput have made 'Wittfeld' battery-powered train ETA 180in various liveries
Liliput have made Diesel Baggage Railcar VT 69 in various liveries or VT 62 railcar in various liveries
Liliput have made Railcar VT 137 (same as Brawa) in various liveries with and without additional coaches
There are also lots of other choices that are too long by your requirements and I used to keep a thread on the Forum which suffers from having photos hosted via Photobucket and Imageshake (charlatan hosting sites) and so many images are missing and it really needs to be updated one day.............but may provide some further thoughts (
Railcar link)
I hope this helps
BR
Glenn
