This thread seems a bit cooled down already, but still...
QUOTE
1. I'm looking for a high-quality turntable for under $300 used, and have so far discovered the following:
Thorens 146/147/160/166?/300?/318/320
Dual 505/CS5000
SL-1200MK2
Those are all good options, but a Rega Planar 2 might pop up too for that price. Remember however that you should put aside enough money to buy a new cartridge(or, if the previous one is good enough, a new stylus). The under $50 Grado Prestige Black is an excellent starting point.
Out of your list, the Thorens 316/318/320 are probably the best sounding. (I use a 321 Mk II with an SME 309 Mg myself, so I'm partial.

) The 1200 should be close behind, if it has been cared for properly - watch out for battered DJ decks. The CS5000 should be fine for MM cartridges as well, but the 505 has a worse tonearm than the Thorens 100 series.
As to the other things... what Andy said.

You can find excellent setup info and some theory at Vinyl Asylum:
Vinyl Asylum FAQCheers and welcome to vinyl!