I also have a P-TAC Lower, I haven't finished the build yet, but am happy with the finish and feel of it. I bought a PA-10 upper and BCG, everything functions correctly by hand so far. I have a friend who has built several of the PA-10 and P-TAC 308's and as long as you stick with PSA/P-TAC for the upper and lower, the rest of your parts will be DPMS style.
The proprietary LPK items are covered in their PSA PA10 Upgrade kit. http://palmettostatearmory.com/index.php/ar-10/ar-10-lower-parts/psa-pa10-upgrade-kit.html
If you already have a DPMS style LPK, purchase this kit and you will be able to use all of the remaining items of your LPK.
There have been some issues regarding some aftermarket FCG, but from my understanding very little "fitting" is required to make this work correctly. Just google Geissele and PSA PA-10, and you will find several threads regarding this. I also would not be happy to take a file to my brand new $200-$300 FCG, but it can and will fit.
I will know more as I finish this build, and will update everyone on my findings as I progress. This will never be considered a high end precision rifle, but I have bolt guns for that. I wanted a budget friendly Semi Auto 308 to punch holes at 500+ yards. With the correct scope and barrel I think I will be able to achieve that.