Resist the urge to modify it or "make it better".
If its "all original", that is great advice. If not, its fair game. The rifle I just bought, someone made it "better". I sold the leupold that was on it and am in the process of selling the bases and rings. When that happens, I'll have $300.00 into the rifle and its still worth $1,000.00+. Its all in how you see things.. I just bought a Tikka yesterday and someone did a lot of things to "make it better". I added up the cost of the stock, mountain tactical bottom metal, picatinny rail, and new bolt handle and the sum of parts were as much as I paid for the rifle. Ha ha.. Doesn't get much better than that.. plus the shop owner threw in a new set of Leupold 30mm rings. When I got there, he asked the typical question: "hi, what you looking for". I said a great deal, my typical response, you have any of those? He said, "pick any rifle and I'll make you a great deal". As for making said rifle "better". He can do that without hurting the originality and value of the rifle. A Mcmillan is a great example of that. Just keep the original stock stored safely so he can return it back to original if he ever wants. That's the beauty in these rifles. I wouldn't cut an original stock to put a recoil pad on it. If original, I probably wouldn't glass bed it either. He did good on the deal and can always make his money back. Cool deal as far as I'm concerned. Now I want to see how it shoots and looks when he gets it topped with that Leupold.
