you have most of your answer in your question.
look at What a K40 is about.
a co2 tube: is it what you need (no info on what kind of material you would put on your granite slab)
XY gantry in a box : XY gantry is that large enough ? in a box (security) but creating some restriction in Z.
focal: original 50.8mm on fixed head, changing the lens is not a big deal, changing the head for one where you could increase the distance between last mirror and lens, it is a bit of a chalenge in term of setting your mirrors.
over all cost, well if you want to change mirrors, lens, head, get rid of the box, not sure a k40 is worth it for you. get yourself a PSU and tube (50w is nearly the same price as 40) a good board to suit your software, aluminium gantry rails is the easy bit, a good enclosure to protect yourself (laser and fumes).
Again, I am not sure about your goals, (it’s possible to engrave granit), loading material from the bottom could make sense, but if at any time you want to cut (not granite), you should be aware a granit base will cause troubles, you want the laser be able to go through to not mark to the back side of your material.
just my 2 cts.