If you're going to finance your start up... start smaller if you can. Particularly since you said this is going to be a part time gig.
Are you and your partner going to be 'legal partners' with respect to taxes? Will you both equally share the risk? Does this mean both of you are entitled to equal profit taking? What happens if there is a problem? How do you resolve it?
Make sure you've got all your ducks in a row before you get yourself in too deep. Talk to your accountant/attorney before you start. If you don't have one, find one.
I could go on... but I'll spare you. I may sound a bit jaded, it's only because of experience.
Lawn Lad, Inc.