The Ruger SR9 uses a proprietary magazine.
Try e-gunparts.com or Mec Gar
No, they do not use the same magazines.
I paid $439 for mine at Outdoor World. MSRP is 525
Can't be answered without a DETAILED description of ALL markings, to include label on box.
I have seen very few used SR9 Rugers at my local gun shops. However you can buy a brand new one for around $360.00. That is a pretty good deal if they get some of the bugs worked out.
From the Ruger website: What type of ammunition should I use in my Ruger 9mm pistol? The Ruger 9mm pistols are chambered for the 9x19mm NATO Parabellum (9mm Luger) cartridge, compatible with the U.S. and foreign military or commercial 9x19mm loads manufactured in accordance with NATO, U.S., SAAMI, or CIP standards, including high-velocity, subsonic, tracer, hollow point, ammunition loaded in aluminum, steel, or brass cartridge cases, +P and +P+ ammunition.
If you want a "Rifleman's" rifle then go with the Remington, if you looking for a lower cost but solid rifle that's your Ruger. Both are good choices but the feel, fit, and triggers are different.
50-500 USD depending on specifics
I'm not sure why you want a two stage trigger. There are a number of aftermarket triggers available from Timney, Jard, Rifle Basix and DAYTON TRAISTER for the Ruger M 77 MKII. Most require some fitting for the safety cam. Wolf Springs also offers replacement springs that will lighten the pull slightly. BS Firearms Services
Marlin and Ruger are two.
Timney Triggers are very good and make a crisp trigger for the 77-22 that breaks between 1.5 and 2lbs. timneytriggers.com