If your wrecker doesn't have a wheel-lift, you're limited pretty much to towing only damaged cars. I agree with WillWren & squeeeala, don't bother with a sling truck.
Ideally you want a truck with a wheel lift & sling, and a flatbed truck. You'll also need a second driver (more $$ flowing out). It'* a tough business, and plan on having every other person you tow attempt to sue you for "damages" to their car - especially the DUI tows. Dudes will get drunk, go curb-hopping with their car, get busted by the fuzz and get the car towed, then try to claim YOU did all the front end (or transmission, rear end, oil pan, alignment, etc.) damage when you towed it.
There are a ton of costs to consider. Insurance, fuel costs, advertising, phone service, time, maintenence. Those trucks get abused and things break. When your truck goes down you not only have the cost of repair to deal with, you're also losing money when your truck'* not on the road. If you've never been in business before, plan on being shocked at all the charges you pay for "business services", like your checking account (you get charged for every check you write), phone service (what costs $20/month for residential will cost $100+ for commercial), internet (again, $50/month residential hi-speed, $200/month commercial). Ever price a yellow pages ad?