It can print using only the grey ink cartridge, should you so choose.
I've not noticed any metamerism with the B+W prints, even when accepting the default settings, which I believe uses a small amout of ink from the photo-colour cartridge.
A small amount of ink from the photo grey is also used when printing colour.
If you want really cheap and only up to A4, try and track down an HP 7960.
There's something odd with the link to the 8750 in your first message, it brings you to the HP 8700 pages and little information. Searching for the HP 8750 I found Steve's Digicam review of it and that says a lot more. The gray cart holds three gray inks if the illustration is correct. That will make true greyscale printing possible.
I learned something.
Its price and the consumables (tri-color carts with integrated head, HP compatible papers) may not make it the cheapest solution in practice though.
I see an A4 model the HP B8300 that looks like it uses the same carts and with a price of 300 $. Edit: it is even A3 I see
The HP B9180 and B8800 pigment A3 models are most likely cheaper in use compared to the 8750. A wider range of (matte + fiber) third party papers possible too. Gloss differentiation and in worst case bronzing on gloss papers is the thing that then may be a problem.