As to surplus 303 British, most that I have seen in the last few years has been Pakistani (POF) and it is notorious for misfires and hangfires. The problem seems to be the primers. They can be a good source for bullets though if you buy them cheap enough as I have salvaged quite a few. There have been other lots from different sources but many seem to have been the victims of poor storage and in my experience is also not dependable shooting ammo. The 303 is an easy cartridge to load and I mainly shoot my own reloads in mine. Neck sizing only (and keeping the brass assigned to one rifle) I get good case life with clean, accurate ammo.
As to the OP's post I have never been into sporterizing old military rifles. The closest I ever came to that was tricking out a couple SKS rifles with aftermarket stocks & mags back when they were cheap (and everywhere). Didn't make any permanent mods and put them back in their original stocks when I later sold them off (and sold the aftermarket parts separately, made more money that way LOL).
I have owned quite a few old military rifles that had been sportered years ago. Back when they were cheap and there were many of them to be had. Some had been nicely done, some were quick hack jobs. They made for fun, cheap shooting. Restored a couple to their military form to fill out my collection (usually just repairing a cut off forearm and replacing a couple bands). Some of the hack jobs I cleaned up and made nicer looking sporters out of, the nicer ones didn't need any work. They had been done by someone who had the time and skill to do it right.
Nowadays anyone who cuts up an original is throwing money away in my view. If they have the skill to make a really nice sporting rifle they would be better off just buying a stripped action and building on that. If they just want a cheap hunting rifle many new guns are really very affordable and would be a better choice. Or simply hit the gunshows, pawnshops and gun stores looking for a milsurp someone cut up long ago.