122mkII is an excellent deck, too. Balanced I/O, all kinds of adjustable parameters. Super nice, I think. Good luck!
how would a Harmon Kardin TD 420 compare to it?
Not sure. Never heard the HK decks. In fact I don't know if I've ever even seen one. The Tascam was a staple in the studios back in the day (still is too, I think).
Can't find much info on the HK, but from the pictures and features I did find it looks like more of a consumer deck. Unbalanced I/O, no azimuth/bias adjustments, tape speed, etc...