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Hello and welcome to BYC!Glad you joined.
I too originally wanted a tractor but soon ditched the idea when I realized it wouldn't be secure enough after each move.
Placement of your coop/run is critical. It must be on high ground to keep it dry and it should not be too far from your house to be inconvenient to care for the flock. I highly recommend you run electric to it. That will help immensely with keeping your water from freezing if you can have some source of electricity at the coop.
As to how much to spend, that is entirely up to you.
I never look at start up costs as anything more than coming out of my entertainment budget. And it is money well spent because the entertainment you derive from watching chickens is endless!
Good luck with your plans and building.