Check out GalliumOS. One of the best, Chrome specific, distros around. The question on hardware comes down to what you want. Many of the newest releases of ChromeBooks do not have removable media (M.2 form factor). So, choose wisely up front: hardware that is supported by current releases and storage and RAM suitable to your needs. The latest CB I have is a Toshiba Chromebook 2 with an i3 processor and 4GB of RAM. That model has a removable M.2 and so mine has an upgraded 128GB of storage. It has received app support for ChromeOS and also runs Gallium with full support. You can dual boot these machines easily, which is how mine is setup.

Nick Janetakis had a nice write-up on setting it up from back when the machine was newer, but to give you an idea:

https://nickjanetakis.com/blog/transform-a-toshiba-chromeboo...

I have a Lenovo Thinkpad Chromebook and I would not recommend it mostly because you have to fully disassemble it to unlock the bios. I managed to do something wrong when I opened it and closed it so after a while one of the hinges just broke.

Look for a Chromebook that you can unlock by just removing the battery. Anything else is just torture.

I have the dell 7310. i3 with 8gb ram. Same as me chromebox last I heard(I think he had the i5 model but it’s not really much faster) it’s about the best experience. Flashing his full firmware is super easy