The Model A as well as the Zero version of any revision of the Raspberry Pi have a smaller formfactor and lack the USB end ethernet connectors of the different Model B version. No Model A nor any Zero of any revision will thus fit into the TX-Pi cases. It is not recommended to use them in a TX-Pi setup.
The Raspberry Pi Model A and B were the first version of the Raspberry Pi. They had a different form factor and a different 26 pin GPIO expansion connector. It will thus neither fit into the TX-Pi cases nor can it be used in conjunction with the TX-Pi HAT. It is not recommended to use the in a TX-Pi setup.
The Raspberry Pi Model B+ was the first one in the current form factor and with the current 40 GPIO expansion connector. It is thus the oldest version that fits into the TX-Pi cases and which can probably be used in conjunction with the TX-Pi HAT. While this might work, it is totally untested. It is thus not recommended to use it in a TX-Pi setup.
The Raspberry Pi 2 Model B, the Raspberry Pi 3 Model B and the Raspberry Pi 3 Model B+ all have the right form factor for the TX-Pi cases and provides the 40 pin GPIO connector required to use the TX-Pi HAT. These are all recommanded for a TX-Pi setup. The Pi 3 B+ is the most powerful of these and is thus being recommanded.
The Raspberry Pi 4 Model B is the newest and most powerful version. It has the right form factor for the TX Pi cases and provides the 40 pin GPIO connector required to use the TX-Pi HAT. However, the TX-Pi software may still be having some minor issues with it. Furthermore the increased heat dissipation of the Pi 4 may require active cooling full performance. The Pi 4 B is thus recommanded primarily for experienced users.
The TX-Pi can be used solely with a (big) HDMI screen even though it is meant to be used with a small touch screen. To achieve this the TX Pi can mirror the contents of the touch screen onto the HDMI output. Both displays can be active at the same time and show the same image. This even works without any touch screen connected at all. Since HDMI screens connot be controlled by touch a USB computer mouse has to be used on conjunction with the HDMI output. Furthermore HDMI output is only enabled if a mouse is connected at boot time. Addtionally if a mouse is detected the mouse pointer is enabled and becomes visible on the HDMI screen as well as on the touch screen. This is a rarely used and tested feature and it is recommanded to use a small touch screen instead.
The first versions of the TX-Pi software setup was written for the Waveshare 3.2" touch screen as it has the same video resolution of 240x320 pixels as the fischertechnik TXT controller. This display is suppored by the TX-Pi software and matching case designs are available. These displays are not 100% compatible with the TX-Pi HAT as they also make use of some of the same GPIO pins the HAT does. As a result pressing the buttons on the display will interfere with the fischertechnik IOs of the HAT. These displays are thus only recommanded for a TX-Pi setup if the TX-Pi HAT is not to be used.
The Waveshare 3.5" (A) touch screen has a resolution of 320x480 pixels and thus twice the resolution of the 3.2" version. It is fully supported by the TX-Pi software, the TX-Pi project privides matching case designs and it is fully compatible with the TX Pi HAT. Many cheap copies of this display type are available online. The only downside is the low screen update rate which is sufficient for simple touch screen user interfaces but lacks when it comes to fluid video playback or gaming. This is the display used in most test setups and is supported best. It is thus recommended to use it in a TX-Pi setup.
The Waveshare 3.5" (B) touch screen has a similar form factor as the (A) but has a slightly better image quality. The (B) needs a slightly different setup and special images and setup options are available for the TX-Pi software. This screen may be used in a TX-Pi setup.
The Waveshare 3.5" (C) touch screen is software compatible with the (A) version and the same setup can be used for both. The (C) version has a significantly faster interface and can thus even be used for smooth video replay or gaming. The TX-Pi software includes a configuration utility that can be used to speed the interface up to 125MHz resulting in a 60 Hz refresh rate. Be careful: This screen is sometimes difficult to obtain and offerings for such "125 Mhz highspeed displays" often result in the cheaper and slower variants to be shipped instead. This screen is thus only recommended for experienced users.
fischertechnik has sold and is still selling several computing and robotics interfaces. Most of these can be connected to the Raspberry Pi and can thus be used with the TX Pi. The TX-Pi software comes with drivers for many of these preinstalled and e.g. comes with demo applicatzions for the the Robo LT controller and the BT Smart Controller. Since this is not officially supported by fischertechnik using these controllers with the TX-Pi may not be trivial and is thus only recommanded for experienced users.
The TX-Pi HAT was designed to be used with the TX-Pi. It is actively maintained as part of the TX Pi project and is therefore recommanded as the primary hardware addon for the TX-Pi.