+1 on the lawnsite forum.
Actually, your question brings back some fond memories. My son began cutting grass for pay when he was 8 (with me closly walking alongside him). At the time he used our consumer grade toro super recyclers. He acquired a total of 4 or 5 lawns, at which point when the one lawnmower he was using broke down, we realized we needed a second lawnmower as backup. So, we bought a second Toro Super Recycler. At that time, we also bought a 4x6 trailer, that we paid for ourselves, for our son to use to haul the mowers around (of course, we had to drive as he was still only 9/10 years old).
Then, my son began observing the professional lawn maintenance companies in our neighborhood, and he expressed an interest in getting his own large commercial walk behind mower. We told him that if he earned half of the cost, we would pay the other half. When he turned 11 years old, he had accumulated enough, so we matched his funds with ours and he bought a 36 inch Toro Proline, gear/belt drive with floating deck and T-bar. As I recall the grand total was about $3600, and he chipped in half, which amounted to $1800. We needed that size as many of the lawns he cut had fenced in back yards, and we needed to get the mower through the gate. However, now that he had two small push mowers and one large 36 inch one, we needed a bigger trailer, so my son paid for half of a 5x8 trailer, and we paid the other half.
Over the next couple of years, we acquired several Stihl trimmers, and a couple of blowers, and an edger - heavily subsidized by us, with selected pieces bought entirely by himself.
Flash forward to this year. This year, he bought a second large Toro walk behind, a 48" with Kawasaki V-twin engine, fixed deck, belt drive. Realizing our trailer was too small now to accomodate everything, he paid for half of a 5x12 trailer.
He's got about 12 accounts now, but plans to expand greatly next year.
My advice would be to grow your business carefully. Make sure you have enough business to be able to get a return on all the $$ you will be investing in your equipment.
My son will be 17 in April