+1 for an I7 6700. it'll handle 10 cameras easily with room to spare.
If you already have a win7/10 product key, you can build your own for less than $400. You can also get win10 for free if you have an edu email account.
System Builder - Core i5-8400 2.8GHz 6-Core, SRM-01 MicroATX Mini Tower - PCPartPicker
8400 is equal to or slightly better than 6700 in most applications, I dont know about blue iris specifically but they should be in the same ballpark.