Go with a RedMax 2660TS string trimmer
Then buy the edger attachment for it. The heads are held on with 2 hex screws and the shaft splines right into each head.
I'm not fond of the systems that detach in the middle because you always end up using 2 drive shafts and reducing torque at you business end
But if your cutting new edges, you might want to think about a wheeled edger to get the job done faster and easier. Stick edgers are mostly for maintaining.