Hi Song,
The Studer A810s are very good machines. Of course, there are better machines out there (A80 and A820 to list a few) but for its portability and sound quality when it is working properly, it is hard to beat. For relatively portable catagory of machines, only the Nagra T Audio would be preferable, but I believe Doc B would agree, for its factory repro audio is not to the audiophile standards and not as good sounding as the factory A810 to my ears.
The things to look out for when considering a Studer A810 are (from top of my head):
1) In general if the deck has LED displays instead of LCD, it is younger and probablily has less wear and tear
2) The head shouldn't have any flat spots but I have seen decks with some wears still sweep 20hz to 20Khz
3) Most A810s have been in storage and you need to spin the rollers to hear worn or dry ball bearings. It's a rude way of testing bearing conditions...but it works most of time.
4) If a deck is rather old, red section (>0VU) of its VU meter will be faded not to mention the VU meter lamps not working
5) If a deck is shipped, it is best to remove the reel table spring loaded spindles (like one would remove the tubes before shipping a tube amp)
6) The basic transport functions; FF,REW,PLAY, STOP along with RECORD should be checked out.
7) LOC ZERO is a good way of testing brake condition. Although it is within spec to try up to three times to get to ZERO, a properly working deck should get there at the first try.
I will add moe as I think of them.
Ki