First, my suggestion is to go with all OEM parts. Aftermarket is less expensive initially but most don't quite match the fit and finish of OEM.
I know from experience.
Also, I agree, do the whole conversion at once unless you are absolutely positive that you will never do the front.
That's a lot less expensive to do than doing it halfway, twice. (or 3 times)
I know that from experience as well.
OEM rear .....but aftermarket hood and front bumper (for narrow front).
In the shop right now getting OEM real Z06 fenders and an OEM 427 take off hood installed.