Digital BW, The Print

I'm brand new to printing and QTR, which I want to use to print sepia 
tones and BO method with my 2100-Epson inks.

I have just downloaded QTR and I'm going through the user guide. The 
section on ink density calibration indicates that I need a densitomer.

What the hell is it? Is it necessary? Is it going to cost alot?

Different question (sorry)..if I buy MIS eboni black cartridge, can I 
continue using the other Epson inks and only substitute the black or 
will the printer not function with the 2 different ink types?

Thanks in advance.