Your first year or two will be a learning experience. If you only have a rental 252 to start with, well by golly go do it! You do need to figure out your costs to figure out your pricing. With that info in your back pocket, you need to figure out a minimum price. Up here in the North, I charge $75 minimum and go from there.
All stumps ground out and chase those big roots out as well. There may be a mighty big hole in the ground, but the stump will be gone. I also charge 1/2 the grinding cost to remove the mulch along with disposal (I do not make money on disposal, I break even there to get the job) and I'll even back fill with the cheapest black dirt if the customer wants. I've had 2 call backs on grind & goes for big roots I missed, & while I was not happy with myself, I cheerfully came back and continued to grind.
Don't let the "big time" grinders here on AS get you down. We all started somewhere, and stumpin' ain't rocket science. Its work, and sometimes its hard work. Don't feel threatened if your experience right now is minimal, study the AS forums and get out there and grind! What you do need to be cautious about is trading dollars. You need to make some money while working. Your buddy's land clearing is a great place to start your schooling on stumpin'. Continue with the job, let him know where you stand (ALL of your customers need this info BTW) and learn stumpin'.
Basic tools I have with me at all times (besides the grinder): Pick/mattock to dig around roots and pull rocks, hard rake for pulling mulch away, long handled shovel for the above as well and my wheel barrow to haul away mulch for clean up jobs to put into my trailer/pickup as needed.
And a 660 with a long bar, the most imp tool IMO...
Bob....