If you have a truck that can safely tow 4000lbs then you can easily get started for $6k with quality used gear. Watch the sale section of this site and CL for deals on a trailer and commercial mower. You will need a least 1 trimmer, blower and hedge trimmer, buy these new unless you can score a really great deal used as these are already fairly cheap. Pick up some hand tools at yard sales and pawn shops. Plan on advertising alot in your first year so that you can have a second year. I made some simple 1/4 page flyers at kinkos and passed those out with a 3% return, much better than the nice color ones I did last year for nearly $600. You will need a basic 21 mower nothing fancy. Keep a tight schedual and don't work or peanuts. PM me if you need any more help.
Knowledge is knowing a tomato is a fruit, wisdom is not using it in a fruit salad.