Digital BW, The Print

>   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 ...

Looking at http://inksupply.com/epsonclean.cfm it appears the 740 uses 
1160 type carts.  The 1160 was a very popular B&W printer.  See 
http://www.inksupply.com/ut_kits.cfm?action=find for a list of some B&W 
options that may work well with the printer.

>... I rarely have to print my photo in B&W, 

Using a printer infrequently has correlated with nozzle problems.

My experience has been that the matte paper only inks like the Eboni-6 
and Carbon-6 are more reliable in infrequently used printers.

Assuming the 740 is a 4-ink printer like the 1160, I'd look at the 
solution I made for the C88.  
See http://www.paulroark.com/BW-Info/C88-C13-5.pdf

> ... gotta save money first...

Carbon-6 is the best per dollar.

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

You may not need any other than the Epson driver.

> Do you see any possible problem with inks compatibility?

Clean the parking pad well.  Eboni (MIS MK) and dye are no compatible.  
I use a syringe with cleaning fluid in it for this.  Even water would 
probably work.

Good luck.

Paul  
www.PaulRoark.com