I've got the 800tf (same case dimensions as 850tf) and the 870tf.
The 870 is a great speedo, same footprint as the 800 and 850, however the heatsink makes it quite a lot taller which would make it tricky to fit under the shell of a lot of buggies.
For touring car where your pulling serious amps and would benefit from extra cooling I would go 870. For 2WD buggy I would go with 850 because of its compact size.
You will also need to buy the programming dongle if you want to adjust the advanced settings, update firmwares etc.
No need to worry about fans with any of the Dualsky 8xx range when used in 2WD
