Go with a kit. They include everything you need and its all designed to work together. IMHO, they pay for themselves 10-fold when you need a replacement part. You call one place and get what you need without a single hassle.
FWIW, I would never buy a rig that had a pieced together kit. If you do, you will have no clue what parts are what. I run into this constantly when customers bring it trucks that they bought that were already built. Its a nightmare to go through them and fix all the things the PO did to them.
I guess the point is: Don't halfass the SAS kit. Buy one that is designed to work together. You will thank yourself in the long run.
I have installed at least 35-40 of the kits we sell on customer's rig to date. I would never try to piece one together. You'll spend more trying to piece it all. The Trail-Gear kits we sell are extremely complete and they will work VERY well.