I don't have an answer for you, but when I leased my truck part of the lease agreement said no commercial use of the truck. If you're doing a finance, you might check the terms for phrases like that too.

Also, if you are using it for business, you need to check with the insurance company for the same reasons.

I'm not sure what field work you would do but a used van can be found for $5-10k and would provide security for any tools, supplies, or other things you are carrying. Having a truck is an open invitation to anyone to easily take what you are carrying in the back.