I did the exact same thing. For small scale I started with a small electric RC, removed the cover and put a layer of Plexiglas and used that to strap everything on. Here is a pic of after a failed test run where to ran full speed and caught air breaking some stuff.
This was before I got the Navio2, you are using it right now so are one step ahead of me at that time. Not a fan at all of the pixhawk. Here is the car I used to get going, I remember it being cheaper so this maybe a bigger car.
Also you’ll need rc transmitter, receiver, batteries, balance charger, and all that good stuff. I would grab a few replacement parts as if you break anything you’ll be waiting on two week shipping. If you have the extra cash buy the RC itself local from someone who has replacement parts. The front steering knuckle broke and since then its been “out of stock” for months on hobby king. Its what you get from buying cheap, disposable RC cars.
Also on the RC transmitter I was caught off guard by how handy it is. I use a 9 channel and having the extra channels to do things are supper handy. So spend the cash on getting a good transmitter/controller. I’m using a Taurus 9XR Pro and have been super happy with it. Channel mixing is super simple.
The Nano won’t be used till you start wanting to interface with your PLCs, if at all. Some of the tractor guys are communicating directly with the PLC and if you are in luck that would be a way cleaner implementation.
Since you aren’t using RTK and just using the GPS you’ll need to realize you’ll get within a few feet of your waypoints, The way ArduPilot does waypoints works well for hobby and aerial vehicles, not so much for my application hence roll my own. I have a feeling you’ll be wanting to do that before its over.