Great point Mark (about the vibrating string). Remember when choosing string guages...you always give up something to get something. A light guage string will be easier to fret but as Mark pointed out will vibrate more and be harder to keep in tune.
Mike's point about not messing with the guages once you've set up your guitar is excellent advice. This applies mainly to people who are striving for "perfection" or want to get absolutely the "best" from their instrument. Ya don't have to be such a nurd about it to have fun, but A WELL SET UP INSTRUMENT WILL BRING OUT THE BEST IN YOU.
Instrument buying and selling:
If you spend 250 bucks on a Korean (or Chinese, or Taiwanese) guitar, you'll get between 50 and a 100 bucks when you sell it (even if it's only a year old. Japanese guitars usually hold about 50 to 60% of their initial sale value (this has gone up a lot in the last few years)
American and Canadian made instruments such as Martin, Gibson, Taylor, Collings, Guild, Santa Cruz, Larrivee, Dunn, Laskin, and a few others will not only hold most of their value but may (not in all cases) appreciate in value by the time you want to sell or trade. Newer brands like Takoma or Seagull, may do the same thing but they'll take another 5 years or so for word of mouth to put them in the same league as the others.
The sturdiest guitar? In my opinion, the big Yamahas from about 10 to 15 years ago are pretty well indestructable. (and often sound as good as the best Martin) Guilds are built like tanks as well. The Collings instruments are gorgeous and sound fine...but I wouldn't tour with one 'cause of their beautiful finishes...too easy to scratch up.
Best, sturdiest, most reasonably priced 12 string? Gotta be the old Guilds. You can find them at ridiculously low prices for what yer gettin'. Often in the 400 dollar range. Warning though, get a neck re-set before passing it on to your grand kids.