Any phone on the Graphene list will work--andi it'll continue to work, even after support from Google and Graphene end...so for "trying it out" you can simply purchase the oldest supported model.

HOWEVER--I agree with buying the best (latest) you can afford. While the 6 and 7 series are good price points, the 8 has imroved security hardware...so that may be what I'd recommend.

AND -- having said all that -- remember Black Friday is not far away, and Google typically has a great deal on an older pixel at that time.