The 7mm mag should get 1000 rounds easly if its not abused to bad , I have heard of guys getting over 2000 before the accuracy goes down hill , and the accuracy factor is a big part of how long it'll last , if it shoots 1/2" now and it opens up to 5/8" and you think thats not good enough then it may be less than 1000 rounds , but if 1.5" groups are ok then you may get 2000+.
As for the caliber that all depends on what your wanting the gun to do , do you want to punch paper at 500yds or shoot elk at 1000yds? The curent caliber you have is a great round and will take deer out to 1000yds if the accuracy is good with the right bullet. Any of the regular mags like the 300 win , 338 win , 264 win and any of the wildcats off of those cases will work with no mods to the action , you can step up to the Ultra mags with some minor action work like opening the feed rails and bolt face adding a Wyatt's box would be a good idea to.
a 25-300 win mag with a 28-30" barrel , 1-8 twist shooting the heavy for caliber Wildcat bullets would make a good long range paper and deer gun , using good quality brass from Lapua you could probably make 100 rounds last the life of the barrel. If your wanting a bigger hammer for longer shots on bigger game like elk then a 338 Edge or 338 Lapua or the wildcats off of the big RUM or Lapua cases would be a good place to start.
but most of all you have one hell of a good base to start with and it should be an accurate gun , get some 160-180 class match bullets and see what she ikes to eat then start shooting so you can wear out that barrel and help support one of our great smiths here.
