I found a 98 Mauser 458 Win Mag barrel for sale and was wondering if this would be a good starting point for a DG rifle?

It sure would Daniel. The .458 WM is often critisize, but it works very well if handloaded.

The 458 WM is a great round and is a great DG caliber. The Mauser action is a great choice for the build. You will be happy with this choice.

So long as a GOOD gunsmith gets the rifle right, a well done Mauser 98 action barreled to 458Win is among the best DGR combo's going. The powders we now have avaialable to us enable the 458Win to achieve the bullet speeds it was originally designed to hit. No need for a long barrel on this caliber either. 20-22 inches is plenty to burn the smallish powder charges in a big 458 Diameter hole. As a point of caution, do not build it too light, as a lower weight 458 will thump you, and good. I would reccomend 10lbs at a minimum, my 416 rigby comes in at 12 with scope and sling on, loaded up and ready to go.

Your probably on the right track if its a good Mauser, it would be nice to know which Mauser you have before I would give it a clean bill of health..What are the markings, that should tell us what we need to know...

The Mauser is also a dandy for the 458 Lott and you can almost do that conversion with a fingernail file and a short poke with a reamer.:):)

Ray the barrel is a whitwhorth 98 mauser it for sale now and I'm considering it.

A barrel is a good starting point if you are willing to spend the $$$ to complete the project. Building a rifle can cost more than buying one unless you can do the machine work yourself. Not trying to discourage you but be sure to do your research first.
That said, I was looking at the M98 (military 1898)barreled actions on this site before I found my CZ.
http://itdcustomgun.com/_wsn/page2.html

I think this is going to cost you less and give you a better starting point for what yo want. Let me know if you might prefer one in wood.
Frank
http://forums.accuratereloading.com/eve/forums?a=tpc&f=2711043&m=1891096501

Frank has a good point. Also, the Interarms actions are pretty much FN clones and are well made.

Gunsamerica.com has had for some time (a month maybe?)an Interarms Whitworth Express Rifle in 458Win asking $950

A Whitworth at that price point would be hard to beat unless you have lots of time and money and patience and really really want to go the custom route. While they are a tad big and heavy for a short cartridge like a 458Win, the CZ 550's are one of the best values going in a true MAuser action and you could ream it out to take 458 Ackley and then use those or 458 Lott or 458Win ammo interchangeably. These are made in 458 Lott as well and are pretty hard to beat right out of the box. The rifle in my avatar right to the left of this post is a CZ, but mine is modified quite a lot.

Well, it save from me buying it. But if I ever need an Elephant gun then I'll be in the market