One piece of advice: buy the Indian bikes. TVS Star2 or Bajaj Boxer are good choices. Very economical. These Chinese things guzzle too much and that affects profitability. The most important thing however is to find a trustworthy rider. Assuming you get into a flat rate agreement with him so that he gives you KES 300 everyday, which I understand is the average (at least where I live), you can expect to break even in about 8 months. Maybe 10 or 11 if you factor in servicing costs. Biggest challenges include not getting your agreed flat fee on the basis of (possibly faked) arrests, "kazi ilikuwa mbaya" and so on. Also, an unscrupulous rider can sell your good parts and replace them with bad ones. Make sure you insure your bike too and hire a licensed fellow to operate it. It'll help you stay out of trouble with opportunistic law enforcers.

After considering all that posted above, unless you get an insurance company that is ready to cover injuries to the rider and passenger ( more so when the passenger is not wearing a helmet) then stay away from this business. One accident can leave you counting your roadrunners or dog Bruno as the only assets you could call your own.