Regarding going with turnbolt - you have chosen..... wisely. A turnbolt is just the cat's meow in a field/hunting rifle, in my view - the perfect balance of weight, accuracy, strength, extraction power, reliability, ease of cleaning, and versatility.
• Winchester Model 70; either Extreme Weather SS or Ultimate Shadow SS (I prefer the stock on the Extreme Weather SS but I'd rather the non-fluted barrel of the Ultimate Shadow SS
Regardless of model, I'm a huge fan of new FN-made Win 70s. Keep this high on the list. I believe the "controlled round push feed" really is the best of both worlds as they claim.
• Sako 85; Hunter Laminated Stainless, Varmint Laminated Stainless or Synthetic Stainless (though the price is stretching it and I'm not sure if I'm comfortable with the detachable box magazine)
Also a great choice. Among your choices, I'd put this at #1.
• Ruger M77 Hawkeye; Predator, Varmint Target, Laminate Compact or All-Weather
Built very well. Has a 3-position safety like the Win 70. Don't know much about them beyond that. Some have said they're less likely to be tack-drivers than some other premium brands - more of a chance of a mediocre shooter, some say.
• A high-end stainless steel Remington 700 (no idea which models are considered high-end though)
Go here and take a gander at the many models of the 700 (some there are shown as discontinued, but most are not):
http://www.remington.com/product-families/firearms/centerfire-families/bolt-action-model-700.aspx
The high MSRP ones are the high-end.
Roughly equates to feature set desirability.
Take a look at the "CDL-SF" for starters. For a real weather warrior, take a good look at the "Mountain SS" - quality non-wood stock, SS metal, and is offered in both .308 and .30-06. Many other options there aren't offered in .308. Rem does not have nearly as many model options as they did just 5 years again. The competition is beastly in turnbolts.
• Zastava M70 PSS (if I could find one here in Canada)
Sorry, don't know anything about them.
1. Can a detachable box magazine be converted to a hinged floorplate magazine and vice versa?
With money all things are possible. But as mentioned, the ease/cost depends on specific brand and which direction you're going - what are you thinking of specifically? I would strongly recommend against a detachable mag and in favor of a hinged floorplate.
2. In general, is installing an aftermarket stock a difficult process?
Depends. Major brand, no - it's easy (Win 70, Rem 700, Rem 7, Browning A-Bolt, Ruger 77, Weatherby Vanguard / Howa 1500, Weatherby Mark V, CZ 550). Typically just drop in, but depending on stock, might need inletting or finishing, and as always, may need some bedding work to make it shoot. Without a major brand/model, it's only difficult in the sense that it may be hard to find one to buy that fits it.
In the end, if you go with Sako, you'll have pride of ownership and quite likely a tack driver (rifles of any brand are ultimately, inevitably, crapshoots to some extent, on that issue). I'd get the Sako and be done with it. Unless a specific model 70 Win set of features appeals to you more. There are many varieties of each, as you know.