Digital BW, The Print

Hello there everybody,

  I have this old printer that I would like to convert to B&W. 

I visited shops and saw many cheap printers, but my 740 is almost
unused and I'm not searching for "the quality #1 in the world". I
rarely have to print my photo in B&W, but I'm not a pro, then gotta
save money first...

What software could I buy to use "1 black + 3 gray" with this printer?

Do you see any possible problem with inks compatibility?

Many thanks for your answers!

Cheers,

Stefano