People collect everything firearm related. The brass unit disks(they're usually stamped with whatever unit the rifle was last issued to) are around, but you don't see them alone very often. Mind you, I haven't been looking either.
"...have to watch out for..." Check the headspace before you shoot it. You should slug the bore too. Hammer a cast .30 calibre bullet through the barrel and measure it with a micrometer. Lee-Enfield barrels can measure from .311" to .315" and still be considered ok. Larger than .315" and it's shot out.
Once you know what diameter your barrel is, you can use the closest diameter bullet you can. Most commercial factory ammo uses a .310" or .311" bullet, but there are some using .312". For reloading, you can get .311" and .312". Come Spring, you'll be able to get 200 grain .313" and .314" bullets from Steve at .303british.com."
Otherwise as long as the headspace is ok, any commercial ammo will be fine. Lots of moose have been put in the freezer by hunters using a .303 of one model or another.
Oh and the bayonets are pricey and mostly useless, but they sure are fun.
