If it really is a good deal, get it.
It can be rebarreled. Since it's a short action with a magnum bolt face, all the WSM or SAUMs are candidates. There may be a spacer in the magazine and a different box or follower spring combination. These are not expensive.
It could be rechambered but all the chambers I know of require loading/reloading your own ammo. If you were going that route then the 25 WSSM would not be an issue. I have only a few 25 cals but I am fond of all 404 Jeffries based WSSM, WSM, Noser and RUM chamberings. FWIW: to me/IMO the 25 WSSM is quite fine for the small to medium "big game" of North America. You got white tail? Ferrell pig/hog? Free roam "exotics" like Audad? Might be worth jumping in to reloading.
If you go the order a barrel route. For chambering, if it were me, 300 WSM or 270 WSM.
Just in case you do want to go down the loading/reloading rabbit hole and keep it 25 cal: 257 Lang is basically a 257-300 WSM. Rechamber with the above mentioned spacer/box/follower/spring update. I don't have one of these, I use a 257 Weatherby Magnum in that role but not a candidate for this build
The first SERIOUS reloading manual on the web.
ammoguide.com
or
Go the 6.5 route with reloading and go 6.5 on a WSM based cartridge like 6.5-270 WSM or 6.5-300 WSM. The difference is in the free bore. See:
The first SERIOUS reloading manual on the web.
ammoguide.com
The first SERIOUS reloading manual on the web.
ammoguide.com