If you can afford a specter dr, it is in my opinion the best fit. Its like they were made for each other.
The GHW Flex. In my opinion they are a great add on. Stops gasses from coming under your eyepro and adds a sling qd soket that I use as my rear mount. Get it when you can, but no reason to put off shooting in mean time.
The GHW Razorback. I installed my short rail last night, must say it is a must if you are running any AR height magnified optics. I had trouble getting a proper field of view through my Specter DR prior to getting the rail. Now it settles in at just the right height when I bring the rifle up. Feels natural.
The GHW TMF. Neat idea, but I turned around and sold mine immediately after receiving it because I did not e the way it changed the angle of the fore grip area. I just ordered a Midwest Industries XL keymod handgaurd the same as TV-PressPass'. You can also install a light internally in it as well, and It gives you the ability to do a more traditional AR/American/C-clamp grip. We will see how I like it.