I have seen several Walkbehinds in Dayton and Cinci Craiglists, usually in 1200 - 1500 range. I bought my 36" Exmark on CL for 1200, and love it. Leave a great cut, and cuts amazing. 13 hp Kaw does wonders. I also picked up a commercial 22 John Deer that runs off a Kawasaki engine as well. picked it up on CL a couple years ago for $100 and though i dont use it much, its a great little mower to have.
I have carried about 5-6 lawns a year for over 10 years, mostly having only a 22" push mower and a 42" Sabre riding lawn tractor. I picked up a Church contract (roughly 6 acres) a couple weeks ago and decided it was time to go big and got me a new 52" ProCat. 0% interest for 36 months was pretty nice feature, besides being an awesome machine! I have a full time job, but have 3 days off to handle my accounts. My goal is to get my mower and truck paid for in the next couple of years, and start working towards totally supporting myself with lawn care, and possibly branch into some plowing in the winter was well.
So though im not as young as you, we seem to be tracking similar paths! Holla at me if you got some questions!
2003 Chevy Silverado
2011 52" Bobcat Procat 22HP Kaw
36" ExMark Walkbehind 13HP Kaw
22" John Deere Commercial Push
Echo SRM-230 Trimmer
Stihl BG66 handheld Blower