i dont know what your clients yards will be like, but getting a 36" allows you to get in the back gate. i dont know if you have big wide open yards or smaller suburban yards. Alot of people like exmark. I got a lightly used exmark metro 36" for $1500.
$3000 is a very tight budget, but not impossible. If you have a truck already that is good. You could check out stihl kombi's. alot of guys use them on here including me. so far so good. it is basically a power head with interchangeable attatchments like string trimmer, edger, pole saw, hedge trimmers, blower, etc... alot of guys reccomend the km 130R the strongest unit.
good luck man