It's killing me to even have to buy a cassette deck. And it's killing
me to not be considering a state-of-the-art machine (if such a thing
isn't an oxymoron), but it just seems ludicrous to spend $1100 on a
122mkIII or whatever just to be able to transcribe 20 year old tapes.
So I'm looking for a deck that will sound good on playback, be able to
record decently for the 1 out of 30 clients who still require it, and
hopefully last long enough to be the last cassette deck I ever have to
buy.

I don't know about the Sony.

It seems you already know about the Tascams.
The MkIIs are as good as the MkIIIs, except a bit cheaper. _Everyone_ wants
the MkIIIs. ;-)
I seem to remember the transports of the MkIs having problems.

Some people have suggested Naks. Don't bother with them, unless you enjoy
constant maintenance. They are good for caring audiophiles, but hopeless for
the daily grind of a studio..

Have you considered a Studer Revox B215?
These were favourites of many studios because apart from sounding fabulous,
they were indestructible, requiring little maintenance.
Big and _heavy_, about 20kg (AFAICR). Unlike Naks, these German beasts were
built to last.
I owned one until about two years ago.
Thinking I would never need to deal with cassettes again, I foolishly sold
it.

-m