I've had a mkII for years. Its a decent machine. I liked running it on high speed, I thought the increase in fidelity was significant.
In terms of build quality, I don't think those late model cassette Portastudios compare to the earlier Tascam machines. I compared a 424 mkII side by side to a 464 Portastudio, and the 464 won hands down. It was obviously a much more solid piece of gear. The really early Portastudios like the 246s look even better, though I haven't been able to do a side by side comparison with the 424mkII.
All that aside, you should be able to get some great sounds out of a mkII.