If it's just for trying out, then go for the cheapest second hand Pixel that's still supported by GrapheneOS and still has a battery that can hold charge for as long as you need it to for testing.

I bought a second hand Pixel 7a for my recent migration. Battery isn't great, but it's good enough to get me through a day.

Battery life improved a lot with the 9th gen Pixels other than the Pixel 9a due to the dramatically more efficient cellular radio. Set it to "4G" or the GrapheneOS added "4G only" as the cellular network mode and you should save a lot of battery life compared to having 5G enabled all the time. Battery life heavily depends on the apps, network and overall configuration. It's easy to end up with bad battery life with lots of apps doing their own push, etc.

GrapheneOS users tend to avoid using Google services when possible and this has a battery life cost when using apps like Signal with their own push systems. In the case of Signal, Molly is a fork with UnifiedPush support that's more efficient but it requires running a server to convert FCM to UnifiedPush since Signal doesn't support it.