Not saying get a #1 for a donor, Ruger chambers the #1 in 458 Lott, I am saying get a factory rifle and be done with it. I have built some bolt rifles with match grade barrels and fully accurized Rem 700s and a couple full custom receivers and to be honest, they did not shoot all that much better then my factory #1 tropical in the Lott. Why?
Mainly because the 458 bullets are not made to match grade specs. Some of them shoot very well but most of them are made for close range hammering of big game with the focus on bullet integrity over match grade consistancy.
Not saying they do not shoot very well but its hard to see a huge improvement in accuracy over most quality factory rifles and a custom when chambered in the big bore rifles such as 458, 475 and 510.
Again, my recommendation was to order in a Ruger #1 for around $900 or find one on the used market for less and go with it. If you look at the Mauser option, say you get a good quality receiver for $300 which in my opinion will be a very good buy for a magnum length Mauser as it will have to be a commerical mauser more often then not. Then add a match grade barrel. My idea of a match barrel is a Lilja, Krieger, Hart, Broughton, something like that so you will pay $300 for the barrel. Now your up to $600. Add $150 to $200 for fitting, chambering and finishing the new barrel and your up to $750 to $800.
Then you need a stock, low end, $100 for some of the wood stocks and bedding which for a magnum chambering should be pillar bedded, $100-$150. That will generally put you to around $1000 to $1050 for the rifle and then any additional misc things such as trigger jobs, minor mag box fitting and such can add another $50.
Then add a base for a scope if the receiver is pre drilled your only looking at around $25 for some of the bases. If the receiver needs to be drilled and tapped your looking at around $40 so your over $1100 at this point.
If you want open sights to be fitted to the barrel, likely you would be over $1200 for the finished rifle.
The ruger comes with rings and open express sights on the #1 tropical so its a pretty good deal and while a very well done Mauser is a nice looking rifle and very traditional, in my opinion, so is the look of the Tropical #1.
Certainly your choice, just offering an option that would be much quicker and in the end likely just as good of a shooter and likely for less then you are going to pay with the Mauser unless you have an in with the smith and he is doing alot of the work for no charge.
I just looked up the CZ American Safari Magnum and you could get into one of those for $1100 to $1600 as well depending on how fancy of a stock you wanted.
Anyway, just offering options that may be quicker and no more expensive then a custom mauser. Certainly not opposed to custom rifles. If I were I would be out of business but I am also not in the habit of spending money that will not produce a return for that investment and I have not seen that a custom rifle in these big bore chamberings shoots noticably better then a quality factory rifle, certainly not like the advantage the smaller calibers have in accuracy with a custom rifle.