The Pixhawk 2.x flight controllers are outdated because they use the STM32F4 MCU, which has limited performance. The key is to look for a flight controller with 2Mb flash memory, as that will get you the full feature set of ArduPilot. See Choosing an Autopilot — Rover documentation for a list of supported flight controllers.
Any flight controller with a STM32F4 or H7 processor and 2Mb of flash memory will last a long time.
That would work fine. There should be some US resellers around, but I don’t know of any off the top of my head.
Also, see Big GPS Round Up for a recent review of GPS modules