Were you looking at .303 because you thought that by default, they would come cheap? Not so much, unless you are more interested in a sportered milsurp. Which you say you are not. Keep in mind that there were a lot of military actions out there besides the Lee Enfield, that were available. Ross Rifles, The afore-mentioned P14's, and a bunch more were out there, and reasonably priced these days if you are willing to shop around.
The reason .303 Milsurp rifles were everywhere, was that the raw rifles were cheap enough that the risks of the whole project going to heck and ending up ugly, were less than the cost to replace it. Nothing more magical or mystical than that. Helped by that minor factor of being essentially the standard Military caliber in most of the Commonwealth countries, for two wars. Lots of it around.
Funny enough, guys seem to be wanting the guns in original condition now, and, as they were readily available and cheap in the past, there are far fewer of them in unmolested form.
With regards to my earlier comment about the wooden ruler, do a little research. Look up the specs for minimum and maximum dimensions for the ammo, and compare those dimensions to the minimum and maximum dimensions of the chamber, and do the same with a more modern cartridge, like .308 Winchester. The differences in size for the .303, are HUGE, by comparison. They were made that way, so all the rifles would continue to function under really adverse trench conditions in wartime. These dimensions are also responsible for the very short case life of the .303, when they get fired, resized and reloaded, and fired again. It can be worked around, but it is still a minor PITA.
Shocked by the costs of Doubles? Most are essentially hand made, to order, by VERY skilled craftsmen. Such, pretty well, has it always been. It's not very hard to spend a hundred thousand dollars on a double these days. Mind, it's not hard to spend that on a pickup truck these days either. The pickup truck stands a far lower likelihood of selling on for the same or more money than you spent upon it, though, something that you can almost count upon in the case of a decent double rifle, if it is well cared for.
Cheers
Trev