Thanks for all the replys.
I see now that I could have spesified a little more, as you can not read my mind. 
The wiering is no problem. Or it is actually prefered as I can have only one powersupply for all controllers.
The RS-485 would in my case be an unneccesary harware layer. I dont think my envirement is that noisy, and it is just a couple of meters wire max. If I have some trubble with noise, I wil try using shielded cable.
The SPI is fine in regards to wireing. I was thinking about the code, sorry for not beeing clear on that.
SPI might not be more conplicatet that I2C or UART, but as far as I understand both I2C and URAT have dedicated input-buffer in hardware. SPI does not?
So the SPI-code needs cearfull timing with interrupts, atleast on the slave side. There might be the same on I2C and UART, but then it is hidden in the library some where.