I attempted an aftermarket hitch this weekend. I got a Drawtite hitch Class II rated at 3500 lbs, without the wiring harness. The heaviest thing I would tow is a 16ft Hobie Cat so that rating is sufficient. Mostly the hitch would be used for the family bike rack.
If you get the 5000 lb rated hitch you'll get extra brackets to stabalize the bumper.
I bought the hitch online and ended getting it with the wrong bolts! The company is going to make good and get me the right hardware. That aside the installation is super easy the rear bumper fascia comes off with four small bolts. Then you just do a simple bolt-on installation. When, and if, I need the wiring harness installed I will leave it to the pros.
The Mercedes Kit with is $300. (5000lbs) (With harness)
The "Draw Tite" Class II reciever (3500lbs) is $90. (No harness)