Remember to buy the older generations so you don't have to deal with 1.8V chips (read the FAQs in the link above). I haven't figured out the 1.8V stuff yet.
The Acer C720 is extremely cheap and very well supported. I'm personally using a Dell Chromebook 13 7310.
I remember those having all kinds of driver issues and weird custom hardware that hamstrung efforts like this...
https://wiki.galliumos.org/Hardware_Compatibility
You can do the coreboot firmware without the ME mod entirely within the Chromebook itself without needing a Raspberry Pi as an external programmer. Only when you want to eradicate the ME do you need the Pi.
Hardware wise: It's Ivy Bridge. You can flash an Ivy Bridge with Raspberry Pi+Flashrom because its chip is 3.3V.