I have an M320 I am modding. $800 is definitely too much. They are old and will probably need a re-cap no matter what. I would try to stay around $400 and less. Beyond that, I think there are better boards out there. I got mine for way cheap....
The design is pretty standard, nothing fancy, decent routing. The 100mm faders and bypassable EQ is very cool. The EQ is pretty decent though.
New Opamps and caps and such in the output section makes a huge difference. I have gotten through a couple of channels, and they do sound better than the stock channels (OpAmps and caps), but not sure if it is the caps or the opamps that made the biggest difference. It isn't the quietest board out there, but I use it for mixing with no problems. With the new output section it sounds great with some mojo.
There are lots of JRC5532's which to me are better than the JRC4580's you find in just about everything else. Then since it is through hole, it is a great platform for modding and you can get the schematics from tascam with the service manual.
The board is also very large for the number of channels, but it does look cool
Considering how cheap you can get the boards for, it is pretty cool.
Love or hate, the boards back then were made to last with pretty good components. I have some big plans for mine, we shall see how far I get without breaking the bank.