In the link section there are a number of MDT manufacturers listed, check those sites for info.
Popular MDT chassis's are the Freightliner FL60 & Fl70 and also the International 4700 seems to be used alot. But there are many other options.
On the transmission issue, everyone has their own preference. I prefer auto and the Allison World transmssion is one of the best, but also big money. There is also a semi-automatic, you clutch to start and stop, the rest of the shifting is done by computer. The MDT manuel transmissions are usually syncronized 6 speeds so they are similiar to what you find in a pickup. The bigger Class 8 trucks (like mine) have a non syncornized tranny so your doubled clutching skills will need to be good. I really would like a auto for my application, just need to find a salvage yard with a world tranny at good price.
Check out
Escapees MDT forum they have alot of great info going in there
Also check
MDT FAQ
1995 FL FLD120, M11, custom interior.