There is nothing wrong with using residential grade equipment (mainly when you start out), it keeps your start up costs to a minimum and prevents you from getting into debt at the start...especially should your business not work out.
You said you already have the craftsman equipment, which I will guarantee you won't deliver the quality and efficiency that a commercial machine would give you, however, that's why a craftsman costs $1500 and a Toro costs $8000. Sure the craftsman may cut decent when you use it on your own home, but after doing multiple lawns a week things start to break and the quality will suffer without proper and frequent maintenance and repair.
You said you would like to get about 5 lawns to cut each week, I'm guessing you won't be driving a truck and trailer around, I assume you'll just drive your lawn mower down the street? 5 lawns is not that many...even just doing this part time I would look into about 15 lawns a week and set that as my goal. I understand you want to keep the work load manageable as you "test the waters" in this industry but you still need to make a little money for the business, especially as you just start up.
My suggestion would be to use the equipment you have while you can. Upgrade your equipment as you see necessary and as you can justify it. It is hard to justify a purchase of a commercial mower with just 5 lawns a week...much easier when you have 15-20 a week. Look for some used commercial equipment as you upgrade, it is much more cost effective than buying new.
'07 Chevy 2500HD
2010 John Deere Z925A 54" MOD
2011 John Deere Quik Trak 647A
John Deere walkbehind