Is there a silencer for the cowboy repeater and a scope for it as well? Is there a long barrel mod for the .327 pistol or is that the magnum?

No. .357 Magnum.

http://fallout.wikia.com/wiki/Fallout:_New_Vegas_weapon_mods

I would actually like someone to spell this sort of thing out for me in words of no more than one syllable: I am in the UK and the only gun I've ever seen IRL was my air rifle (aka a BB gun), so I am having trouble determining which weapon mods go with each weapon, and what ammo can be broken down to provide parts for other types of ammo. Assume I am a baby with no knowledge whatsoever about weaponry (which is somewhat true) and explain it all to me - I'd be very grateful!

The ammo reloading system is actually very unrealistic. With extensive knowledge of guns/reloading, it is in fact seems even more unrealistic. You can look up a tutorial online probably, but it has nothing to do with real life gun knowledge.