If your going to embark on this offer here is a couple of things to consider...
* Are you bonded and insured? I don't think I need to say anymore than there is a high risk of property damaged, even if you don't hit anything your still going to tear up the lawn...
* Does the owner know that if you leave the stumps in the ground walnut will sprout up again next year? Will you tell them, if they want them gone the stumps will need to be pulled up, leaving big holes in their yard.
* How will you deal with clean-up? Do you have a chipper? and a box truck to haul away the chips?
* How will you load and transport logs? Do you have the right equipment?
* Are these yard trees? If so don't "pay" any more than pallet grade / Log grade 3 or firewood / log grade 4 for the logs. Why? high chance of steel and usually low quality / wind shook lumber, sometimes you get lucky but not very often.
* Make sure you have a written and signed contract - have your attorney approve it...
* Better yet - consider hiring a professional to take down the trees, load and deliver the logs and cleanup - Several Ben Franklin's will be cheaper in the long run than a DIY job.
Good Luck with all that...