A RIS or RAS won't eliminate barrel wobble, it'll just prevent you from handling the barrel directly thus loosening or noticing the wobble. I had great fortune eliminating barrel wobble using a G+P style delta ring. They lock down TIGHT and fix everything in place.
Most after market hand gaurds simply replace the plastic ones along with the removable carry handle and require little or no modification to install.
Do a google search for M4 handgaurds and enjoy the ride, there are hundreds of options. Most real-steel handguards will work with the Marui M4 as the rails are compatible size, but double check if you aren't sure, someone in here will know specifically. For just airsoft stuff, look for ideas at
www.UNcompany.com or
www.WGCshop.com