Just search the Apple site; you'll get any system up to 7.5 for free.
Maybe the Plus is driven best with a System 6, I recommend trying that out.
And check the battery - I think it must fail after so many years.
If it starts up at all, let it run for a while. Hold down cmd.-opt.-P-R to restore the PRAM when starting up again.
If that works, open the cdev's and set the date and time anew.
You'll have fun with this one !
In addition to RescueMyClassicMac.com or MacBootDIsk.com you might also head over to LowEndMac Vintage Forums where Platnicat has offered to make boot disks for free (including shipping on your first disk).