Having owned all of the above, these are my opinions, which should be taken with a grain of salt
SVT 40
Pro: Semi Auto, accurate, cheap, historically valuable, built well.
Cons: Since you're planning corrosive ammo, it will need to be pulled completely apart after each shoot. You can get away with less than a complete teardown, but I found I was getting some corrosion between the wood and the outside of the reciever. Spare magazines cost as much as some guns. Ammo can't be reloaded because of milled fluting in the chamber to facilitate extraction.
Mosin Nagant 91/30
Pros: Refurbs are in good condition and well built... like a tank. Can be very accurate. Cheap guns.
Cons: Not the best built guns ergonomically and design. Heavy.
Carbines
Pros: Lighter than a 91/30, quick pointing, cheap, built strong, easy to carry.
Cons: Not overly accurate out past 400 yards. Basically built for "close" fighting, so that's what you get.
As for a reproduction sniper:
Mosin Nagant with installed scope and bended bolt did it increase accuracy ?
Yes and No.
A scope will help the shooter maximize their ability to shoot the gun accurately, and the bent bolt allows the knob to clear the scope. Within limitations, a gun will always shoot the same, most modifications are meant to help the shooter perform closer to the abilities of the gun.
A reproduction scope and bent bolt on an
accurate mosin will help you shoot better, if you know how to use them.
P.S. Modern style scopes and modifications are not worth it. Because it is a military
surplus gun, no value is added to
non historically accurate modifications. That means if you spend $200 on a milsurp, $200 on scopes and bases, and $100 on gunsmith work, your gun will be worth about $250 tops. This is because the gun no longer has historical value, and you're basically selling your scope and bases along with some spare parts. Odds are if you have to ask these questions, you also don't have the skill to do a good job yourself to save the money that way.
Just a heads up. I know you probably weren't thinking about it, but every so often someone does something like that and get a very rude awakening.