widely available and excellent quality surplus 7.62x54R.
No so much any more, and I don't think I would go say "Excellent Quality" regarding most of the Com-block surplus either. Some is pretty good, but from what I have seen there is as much garbage as really good, with most being OK.
Spam cans are pretty much gone, and when you do see them, they are ~$200+ for a 440 round can. There is commercial stuff out there, probably made at the same factories that made the surplus stuff, but it is not that much less expensive than cheap .308 any more.
Regardless, if you want a milsurp rifle you can find ammo at a big box store, you are really looking at a pretty short list, .30-06 or .308. This is assuming "Big Box Store" means the same to you as it does me, Wal-Mart.
That leaves you these options: US 1903, 1903A3, US 1917, the Ishapore Enfield you mentioned, and a handful of Mauser variants in .308 (Israel, Spanish FR-8) or .30-06 (Norway, Belgium, Columbia, Mexico).
Avoid the Spanish 1916 carbines, they are small ring actions never intended for .308 pressure levels.
If you are wiling to mail order ammo, you may be able to find an original Mosin Nagant PU Sniper for not much over your budget, there were a ton of them imported a few years ago, and they come up from time to time and it already has the side mount scope.
Then again, if you are willing to mail order ammo, that opens up a lot of possibilities on other rifles, particularly Yugo Mausers.
If you are committed to drilling holes in a historic rifle, try and find one that has already been sporterized, probably save some money in the long run.
If you are thinking that you will save money over a commercial hunting rifle, those days are gone. I like milsurps, but even the cheapest ones, The Mosin Nagant 91/30, the supply has dried up considerably, and bottom of the line 91/30s are going for ~$250 these days.
The days of modifying a military rifle to a sporter as a more economical way of getting a quality hunting rifle have passed. This made a lot of sense when they were a fraction of the price of the bottom of the line Remington or Winchester, and could be found by the barrel full in the local hardware store for $10. Those days are long gone.