I am assuming that the M&P 15 model that you have has a fixed front sight base, or you have the OR model with the railed front sight base. If you are only going to attach a flashlight, I would go with a drop in rail (non free float), or Magpul handguard and call it a day... much cheaper and easier solution.
However, if you really want to go free float, there are a couple of options...
1. A drop in free float rail that utilizes your fixed front sight post. Troy makes a few of these, the Charlie, Delta, or MRF. The Charlie and Delta utilize the delta ring on your rifle, nothing to remove. The Charlie is a quad rail, where the Delta looks similar to the rail pictured above. The MRF requires you to remove the delta ring and locks up directly on the barrel nut.
2. A one piece unit that requires removal of the front sight base and replacement with a low profile gas block. Some of these will use the standard barrel nut, and others have proprietary mounts that will be included with the rail kit.