These machines were all pretty much "business" computers, more than anything else. The main difference between them and the others was in the use of 8 inch disk drives. The Model 2 was a Z-80 based machine, while the 16/6000 used a combination of the Z-80 and Motorola 68000. The Z-80 served as the disk I/O processor, and ran the machine when it was booted in Model 2 mode.
Wikipedia TRS-80 Model II entry
Usual type of site for a Wiki entry - stats, basic description of hardware, capabilities, etc... Good starting point for info gathering
Provides a good overview of the system options, model numbers, peripherals, and a brief history as well as reader feedback
This article is quite detailed, including machine description, specifics, options and several images of Radio Shack catalog ads
TRS-80.com (Ira Goldklang) Model II entry
This article provides a very concise discussion of the Model II & 16, including history, specs, hardware options, software... the list could go on
This article discusses a Model II dontated to the DigiBarn Computer Museum
Hackaday strikes with a repair article on the Model II
Not sure about the origins of this site, but there's good info
Not a lot can be said about this entry - it's a brief run-down about the computer, and it's history
PSKI Model II software archive
PSKI GIT archive od software on the Model II/p>
Computing History Model II Article
A brief history of the Model II and it's hardware
This link contains a plethora of PDF documents related to the Model II
This article is a 'how-to' use the Lo-Tech IDE Adapter on the Model II
A short technical article about the Model II and 12
Wikipedia brief TRS-80 Model 12 entry
Very brief discussion of the Model 12 - very little info provided
The last part of the Wikipedia entry is dedicated to the Model 16B & 6000
Provides a brief overview of the system configuration and options
Technical documents related to the Model 12
Rundown of a Tandy 6000 that was restored as a Model 12
Pictures and specs on the Model 16 - Lots of pictures
A very brief article on the Model 16's differences from the Model 12
A short discussion on the various CPU boards in the Model 16 - 6000
This is a Google Groups (yeah, I know) discussion on the differences between the two machines