The second figure of the '6400/9600' simply refers to the precision of the stepper motor, which in reality is just put there for marketing purposes and doesn't really mean anything regarding scan quality. I would take the '6400 dpi' figure with a very big pinch of salt (again because of marketing purposes) though as test charts have repeatedly proven that those figures are massively inflated, and at that 6400 dpi setting the V750 only actually resolves ~2300 dpi. This is just simply because of the laws of physics: the sensor may be able to resolve 6400 dpi etc, but other optics in the scanning path (especially the high pass filter) limit the actual resolved resolution so files just end up massively bloated with no more resolution than scanning at ~2300 dpi in the V750's case! Only dedicated film scanners tend to reach anywhere near their quoted figures.
From what I can work out the V750-M is simply what they call it in the USA as Epson UK only mention a V750, and Epson.com only mention the V750-M. The fluid mount is included by default with all V750's (it also works with the V700), but if you want to scan lots of negatives then its messy as you will have to clean the negs after scanning, and clean the scanner/dispose of the fluid so it could take much longer for not too much gain. If you want to save some money then look for a standard V700 as detailed comparison tests have revealed no actual differences between the image quality of the V700 and V750 (despite Epson's claims), the only difference is that the V750 contains the fluid mount kit and costs quite a bit more.
EDIT: Taking another look it appears that the V750-M (which is only sold in the USA) does include the fluid mount holder, but the standard V750 sold elsewhere does not and it has to be purchased separately.
