Download Older OS

MBP 15" i7 2.3 mid 2012 MD103LL/A

Finally happened, my wife spilled coffee on keyboard, just a few drops she said the next day when I was informed it was acting 'funny'. I opened it up disconnected the power, pulled the HD, and put a fan on it. She was in the middle of a book project and hadn't backed up for a few days. Data on drive was hand transferred to her new MBP and at some point she did something that hosed the OS which I discovered after reinstalling the HD in the old MBP.

I starts up and seems fine but can't find OS. What I need to do is D/L OS. Can I do that on a different (and much) newer MBP? When I finally found where to get them I was presented with Yosemite and Mavericks as having been 'purchased' and avail for D/L in AP store. How do I get it/them to the HD of the old MBP while it's connected as external drive? I have seen messages about 'system being to new' and 'install on this computer'

Is it possible to D/L direct to the external drive, reinstall that back in to the old MBP then run the installer? IIRC the orig OS on this was Lion but that doesn't seem available.

Thanks for any help.