There are several different Kolar off the shelf 'standard' stock's available (the #1, #2, #3, #4, etc), both right hand and left cast. They would all have an adjustable comb, but that is just one of the many dimensions so they may not be exactly right for you. It is possible that the good old #3 will line up perfectly for you, be the right length, pitch, etc., you just do not know until you know. Yes, one of these would be the stock that would come with the gun from the factory at the base price that Paxton would quote you. I assume that Paxton will have a demo stock of more or less each and every one of these stocks available on-hand but that is a huge assumption. If he doesn't, and the stocks he has at the shop do not seem to be almost exactly perfect for you, you might consider calling or have Paxton call back to Racine, speaking with Fournier or Ramagli and inquiring into the possibility of a few demo stocks being temporarily shipped to Dallas for a fitting. Some dealers also have used demo guns available in which you could actually go shoot the gun with the stock. Again, I just do not know if Paxton does or does not, but wouldn't hurt to ask.
A custom stock is going to be custom fit exactly for your body. Perfect length of pull, perfect cast, perfect drop, perfect palm swell and grip, etc. That said, it will likely also have an adjustable comb cut into it so if you gain or lose weight and need a small adjustment it could be accommodated. If you are going to fly back to Racine and have the work done, you'd probably want to have it done with your initial purchase. Doing so would save you the cost of the factory stock and the cost of the custom. At Kolar, Jeff Mainland would build the stock, he is excellent, very experienced and you will not be disappointed; he also owns the company. I asked him what percentage of the new Kolar guns have a factory stock verses a custom fit. He indicated that at least half and probably more like 2/3rds of the owners fly in and have him build a custom stock for them. At least that was the percentage at the time I was there. Due to that demand, having Jeff build you a stock is not inexpensive, but I would do it again. That said, there are other stock makers out there as well…. decision, decisions!