Ok... I had the idea of what I wanted... I just couldn't visualize it. A few minutes googling images, some MSPaint (anyone know a nice simple linux alternative?) and now I can see it:

 - Each rack has a small arduino conencted to sensors and an RF link (NRF24L01?)

 - Central unit will be a raspberry pi. Why? Because I have never used one and this gives me a nice excuse.

 - RasPi will gather data via another rf link and present it on a custom website. A "heatmap" would be a nice addition. Not just plain sensor data numbers...