Well, most of those barrels in that price class are shot out.
But slugging the barrel will not tell you much. Most of the shot out ones are really rusted out from shooting corrosive ammo without proper cleaning, as much as actually being worn out by firing.
This will tend to make the bore bigger and rougher, but its not unsafe as long as you have gotten most of the rust out of the barrel.
You can never tell how these old barrels will shoot, though. You just have to find a rifle you like, buy it and test it out by shooting. There's really no other way.
Other important things that will rob your old Mauser of accuracy are poor stock bedding (lots of these are not even in the original stock made for that barreled action), bad crowns, worn lands and nasty triggers. Slugging the bore won't help with any of this.
The best thing is to look at the bore with a bore light before you buy. Don't listen to the seller, they really fib about bores. If it is dark, it really rusted bad. Look at the lands at the beginning of the rifling. They should have nice crisp edges. If they do not look crisp, they are worn. It takes some experience to start noticing this: what is crisp and what is rounded. You just have to look at a lot of them.
Look at the muzzle crown as well, but this is easy to fix if it looks rough. But if the rifling that goes down into the bore is damaged, you have a bigger problem.
Often the true horror of the pitting and rusting of these bores is covered up by masses of metal and powder fouling. It your new bore seems like it takes forever to clean, it is probably badly pitted and rusted and has fouling down in these pitts that just does not come out easily. After you know what it is, you can actually feel the roughness of a bore through the cleaning rod as you first start to clean it.
One of the joys of a nice bore is how easy it is to clean. The fouling just comes off a smooth bore better than a rough one.
You cannot expect really good bores on these old military rifles. The vast majority are pretty rough by now, people just don't want to admit it. The rifles (especially in the price class that you mention) are now so old and so beat that you cannot expect them to have gilt edge accuracy, there is just too many things that could and probably have gone bad.
Just get one that seems to appeal to you for some intrensic reason and do not expect anything sepectacular from it. Almost all of these rifles will still shoot good enough for hunting and defense and if you are lucky, you will find some that still shoot surprisingly well. But I know of no way to sepparate the real gems of shooters for the run of the mill sort.