Make sure that your existing single speed chainring is 3/32" not 1/8", so that your 8-speed chain will mesh properly. Set your chainline carefully, getting as close to the 4-5 intercog space as you can. That should do it. Not an IGH set-up, right? So, get a rear derailleur that has a snappy tension spring, and that can handle the largest cog size that you plan.