finding out which OS it shipped with

Hi, some simply questions - have looked around the net & these forums & think I have most of the info needed but would appreciate checking & some clearer info if possible:

I have a 2009 macbook pro. I also have a newer macbook pro so want to update this older one.

1) I upgraded the OS to El Captain & there are some issues (the version of imovie not being fully compatible with el captain & Apple charging for an upgrade if you want it to work properly ! + some other bits):

. is it possible to check what version of OS this macbook shipped with someone on the computer ? Is there, in effect some kind of database of the OS versions it has used at various times ?

. i've read various contradictory posts that say either that if you erase El Captain you will then automatically be prompted to reinstall OS with the older version you had & posts that say that won't happen - does anyone know the actual answer & is there a way to check which OS will be prompted to be re-installed if one erases El Captain ?

. if I choose to keep El Captain but want to swap the version of imovie (9) on this macbook with the one I have on my newer macbook, will that work & do I simply save the imovie app to an ehd & then drag it into the apps of the older macbook pro ? Any potential issues I should be aware of ?

2) the macbook only has a small hard drive & 4gb memory - i've found various tutorials online for how to upgrade both & it looks reasonably straight forward once one finds the correct new drive with the right Sata rating & speed etc. On the memory front are the lower priced versions one can find online as good as Apple's high cost versions of the same thing ? Has anyone done this double upgrade & had issues or success ? slightly nervous to actually do it !

thanks,