The main difference between Metford and Enfield rifling is the service life with Cordite. I don’t know of any testing particularly with modern powders. Metford rifles barrels last very long with BP but not very well with Cordite. You can use it with modern powders if it’s suitably “nitro” proofed. Metford rifled rifles is scarce.
With any old rifles, look out for corrosion and pitting down the bores. Ideally you also need to measure bore diameter, some (Especially 303’s) are well used and while the bore might be shiny it may be shot out with an oversized bore. You can still potentially shoot them as long as you load bullets sized to the bore. Don’t mix them with regular ammo or you’ll blow up another rifle.
Throat erosion is very common and may be fine for limited use while you still get accuracy. Pitting near the muzzle end is looking for trouble (in terms of accuracy).
You can significantly extend the service life of an old rifle by using round nose flat base bullets. These are not affected much by wear or light pitting and you can still get good accuracy from rifles in which boat tail bullets will tumble.
6.5x54 is not too bad to feed, you just need a few cases to get started, dies are available.
7x57 is no harder to feed than 303.
Don’t forget about 8x57 and other popular European calibres

Just watch out for bore sizes in 8x57j, again these are scarce but they do pop up occasionally.
Another consideration is whether you want to use a scope or open sight? Most classic rifles never had scopes mounted and should not. If you want a scoped rifle, buy one that’s already ruined.