I hunted buffalo in Zim in 2015 with John Sharp Safaris (http://www.john-sharp-safaris.com/). The hunt was on the Bubye Valley Conservancy, which is 880,000 acres. You'll see lots of everything there, including all of the Big 5. Flying in and out of Bulawayo is very good too. PM me if you'd like more info.
Desfountain and Jones. Ltd. is a great outfit and does hunt buffalo in Zimbabwe along with all of the big 5 and plains game. They have a couple of concessions that are great for buffalo Msaize in the Save Valley also Maninga on the Zambezi Valley escarpment. They usually come to the US for the Dallas SC and the SCI show so you could meet them in person. Check them out and don't be afraid to give them a call or shoot them an email.
Cheers and good luck with your planning!