This is kind of an interesting topic for me as well.

The first bike I refurbished, I spent way too much time cleaning and oiling the original chain, only to start breaking links with in a hundred miles or so. So I got a new chain, only to find that the cogs on the freewheel had worn to the point where there were junk with a new chain.

You can oil and clean your chain all you want, but overtime it's going to stretch -- so here's the question for those of you have been doing this for a while.

Is it the stretching of the chain that contributes to cog wear, since the chain and gears no longer mesh properly, is it an unmaintained chain, lots of dirt and what not that grinds teeth down, or a combination of the two?

Regardless, I've started to take the approach of, unless you know the history, cables, tires and chains just get replaced before the bike goes on the road.