Do you have any chassis recomendations?
When I think large-scale autonomous robotics - I think of something the size of quad-bike or larger.
That said - for that size, you can typically get nice quadbikes or go-karts (IC engine) or golf-carts (IC engine or electric) for only a few hundred dollars; heck, with enough looking around, you might even find someone giving such a thing away (just haul it away for them!).
You'll likely still spend more than a bit of money getting it running again, and then even more setting up a hardware interface to control it (such large machines require hefty actuators for steering, and high-power motor controllers and such - all of which adds up to a lot of money).
One step below this level - and maybe better for your budget - would be to get a Powerwheels (or similar) ride-on toy and convert it - many people have done R/C conversions, and there's a whole ride-on toy modding scene out there (check out modifiedpowerwheels.com for starters). It will be cheaper (still, expect to spend a few hundred dollars over the course of the project).
Below that, you are looking into large-scale 4WD R/C cars (T-Maxx, Losi, etc); still, you'll spend a few hundred dollars again.
Even building your own chassis from steel or aluminium, and low-cost hand-truck or garden-tractor tires, while possible - still won't be inexpensive. Basically, once you move past carpet-crawler size, you can expect to pay a lot of money for the chassis (especially if you want something rugged and capable of carrying a bit of weight).