Brother in Law wants to not get in financial trouble

Knowing and planning for the costs before purchase is extremely important. If you don't consider ongoing costs, you are being careless.

On the other side, if you focus too much on comparing repair costs between rigs, you may end up with a rig unsuited for your needs.

Rather than debating whether diesel or gas is better, if folks would simply provide some actual costs they've incurred, the OP can help his BIL make the right choice for him.