I'm not an expert by any means, but I play, tune, and know a bit about piano repairs and used instrument selection. I'll see what I can tell you.
Yes. A flat soundboard is a good thing.
There's one on eBay from 7 years later and they're asking $30k. That seems absurdly high to me considering that a new Boston costs less and IMHO tends to play a little better than a Steinway of that vintage unless the Steinway has been rebuilt very well.
Don't know if the one you're asking about is a New York or German Steinway. If it is German, you should definitely buy it and get somebody to redo all the felts and maybe replace the hammers. You'll have yourself a wonderful piano for a great price. If it's from New York, your mileage may vary a bit more from piano to piano. $6500 is a very good price either way, though, IMHO. I'd have expected you to say $12-18k from a dealer, though $6500 seems reasonable from a private buyer that hasn't had any part of it rebuilt/adjusted ever.
I would definitely pick the Steinway over any Yamaha in a heartbeat. The only thing about the Steinways is that they are -very- inconsistent. You'd want to try it out and see if the keyboard has a good feel.
Make sure:
1. Action isn't sloppy:
a. a "thump-back" a moment after hitting keys hard is bad
b. pushing the key halfway down before the hammer starts to move is bad
c. keys not level will require some work (though I think that's easy to take care of---usually by replacing felts and/or adding shims)
d. dampers should mute all the notes correctly when you release notes (again, very easily fixed, but it does reduce the value of the piano a bit, IMHO, since you'd probably have to pay to get it fixed)
2. Action is comfortable:
a. should not be too heavy (too hard to get volume out of it)
b. should not be too light (too hard to get soft dynamics)
3. Good tone:
a. Not too muffled (hammers too soft or strings in bad shape)
b. Not too bright (hammers too hard and/or worn down)
4. Listening environment:
a. Make sure that the dealer isn't showing it on a hardwood floor if you're putting it on carpet or vice-versa. (Alternately, consider adding a small hardwood area on top of the existing floor under the piano.)
b. Make sure the room is a reasonable size so that you have some idea what you're going to get.
5. Tuning (used pianos):
a. Find out how recently the piano was tuned. If it has been a while and it is still in tune, that's a good sign.
b. Find out if the pegs have been replaced with oversized pegs. This is commonly done to fix pegs that have wallowed out the holes. If the piano is 50+ years old, it would be a miracle if it didn't need at least a couple of pins replaced. If they haven't done so, you should ask them if they have checked the pegs to make sure they have proper resistance.
c. Examine the bridge where the strings pass over it between the dampers and the tuning pegs. If the bridge is rusted (usually due to water damage), you should insist that this be repaired. Rusted bridges will stick badly and make tuning a royal pain in the @$$. If they aren't willing to do the work to sand and paint the bridge, I'd walk away, personally.
d. Examine the strings to make sure they don't have any rust damage. This usually goes along with a rusted bridge, but you may find rust only on the strings if it is caused by decades of atmospheric moisture rather than water damage.
6. Soundboard condition (used pianos):
a. Except for the support ridges, the soundboard should be perfectly flat, or at least really, really close. If you see warping, this is usually a sign of water damage (or very seriously excessively moist atmospheric conditions). This is hard to fix.
b. If you see splits, this is usually a sign that the soundboard got too dry. This is fixable, but the general consensus from piano repair people I've talked to is that it is usually better to just leave small cracks, since fixing it can often make things worse in the long term. Of course, if you hear it buzzing noticeably, you'll want to fix the cracks.
c. If the soundboard has separated from the supports, you will likely hear buzzing. This should be fixed promptly.
As for your question about it lacking high end, there are three possible reasons:
1. The dealer has it on carpet and/or in a large room and you're used to hardwood and/or a small room with hard walls to reflect the sound back at the performer.
2. The hammers are too soft. Hammers harden significantly with age, so if they just rebuilt the Steinway, it is likely that it simply isn't broken in yet.
3. Pianos when tuned properly are tuned with a stretched tuning. That means that as you get higher, the notes are tuned slightly sharp, and as you get lower, the notes are tuned slightly flat. The norm is about one cent every two octaves, though I like mine at about a cent per octave, personally.
Anyway, if you don't stretch it or if it isn't stretched enough, it will sound dull. I -think- that pianos lose their stretch over time, so even if it was properly tuned at one time, there's a good chance it will have sagged.