Assuming you want a functional rifle, not a full-out custom, I'd suggest you check the E.R. Shaw site's pricelist for rebarreling, bolt forging and drilling and tapping and maybe fitting a Timney trigger with safety. Then find a stock from Boyd's or Bell & Carlson. Add it up and there you have it. Another place that'll do it all is the Williams Gunsight Company.
What you'll probably discover is that you can buy a very nice new rifle for less money and far less trouble and time. It won't be a unique piece built on your nifty little Mauser however, so if that's what you really want and are willing to pay and wait, have at it.
If it's just a nice Mauser sporter you want, Gunbroker has nice commercial ones all the time for decent prices; from basic models originally sold by department stores like Sears to very nice Brownings, FNs, and Husqvarnas, to name a few of the better ones. You can also find good buys on customs built for other people, but really nice ones are going to be pricey, just not as pricey as having one built yourself.
