@Blain
Carefully check the bore at the muzzle for nicks and/or wear of the rifling caused by improper cleaning technique/tools. Edges of rifling at muzzle must be sharp, not rounded. If present and minimal, a simple DIY crowning will fix things. Deduct about $50 from price if re-drowning is required.
With Minis this old and of undetermined use, replacing the Op-rod spring with a Standard Power spring is wise. NO "Extra Power" springs anywhere. Factory Ruger mags for now.
Get (download) Owner's manual from Ruger.com and read it
carefully. Rear sight is adjusted in 1.5 MOA increments, NOT 1 MOA; strongly suggest getting sight adjustment tool, which saves hassle and scratching the rifle.
Clean and lube the rifle using grease/oil as applicable. Watch Mini-14 videos on YouTube from Brownell's--multi-part series. Minis should be cleaned using a pull-through from breech to muzzle; Strongly suggest appropriate Otis cleaning kit, also useful for M1/M1A/M-1 Carbine and other firearms.
Determine Rate-Of-Twist of your barrel using this
CHART , and begin testing appropriate ammo. Take careful notes and expand to different bullet weights as desired. Every rifle is a bit different. Take careful notes. Using
decent ammo will give far more reliable results than using crappy ammo.
Do Not make any alterations to the rifle until and unless you are satisfied that it will not need to be sent to Ruger; verify 100% reliability first, while sighting-in/ammo testing.
Once things are certified to be GTG with the Mini, and you have some info from ammo testing, come back and we can discuss things. Take careful note of how far spent cases are flung, and where they land in relation to the shooter.
If you happen to be not satisfied with the Mini's accuracy, there are many options available, most of them inexpensive DIY items.
Manage your expectations; Assuming a mechanically sound Mini-14, and
decent ammo, your Mini should be capable of around 3 MOA or perhaps better.
Again, report back with info requested above, and folks here can help, if needed.