Congrats Axel on scoring some great Garmin displays for your project!
While there are some lower cost options for the OAT probe (if that is all you buy), you will need some additional parts like ADAHRS and magnetometer to turn your GDU 37X PFD/MFD displays into a G3X system, so you will probably want to buy the K10-00016-14 System LRU Kit which bundles the GSU 25C ADAHRS, GMU 11 magnetometer, GTP 59 OAT probe, and Configuration Module for $1,500.
You only need 1 configuration module (included in this kit) for the whole system. It is installed in the PFD connector backshell.
Here is a link to the pricing list
As Walt said, you will need a GDL 39 or GDL 39R for ADS-B traffic/weather. The first time you use Garmin Target Trend traffic, you will be really glad to have a Garmin ADS-B receiver to go with your Garmin displays.