I can only tell you what i know, and that is precious little compared to what the Mosin gurus over at gunboards.com or
http://pub113.ezboard.com/bparallaxscurioandrelicfirearmsforums
could tell you
hex recievers are condidered more collectible for mosins i believe
not any particular advantage to them just more rare i think
a pre-war manufacturing would probably be better...but then again one made in '45 might never have been issued....its a toss up
everyone will have their own tactics for cosmoline removal, when my dad and i worked on our mosin we wiped off as much cosmo as we could, then took the baby apart and wiped out as much as possible, then you can take gunscrubber or (now i use brake cleaner.....1/4 as expensive) to the small cracks and parts you couldnt wipe clean and it is a good degreaser
i like to give the mosin a nice little lubrication after a degreasing though
we also refinished the stock, and to get the cosmo off you could just spray it with the hose and then let it dry......but we put stripper (for furniture) on the stock and got all the old goop out of it, then neutralized the stock so the new stain we put on would hold
we put on a couple coats of stain and finished it with tung oil and it looks pretty original IMHO
a couple good sites to check out are
www.mosin-nagant.net and
www.russian-mosin-nagant.com
they are great for mosin info
again other folks may have more efficient ways of accomplishing what you want
im kinda new at it myself
BSR