The additional cost of the mouse is unlikely to be significant compared to the drawbacks of having one with poor ergonomics. That, and the tablet stand, are definitely not worth questioning unless they are crazy expensive (at least $300 each with obviously comparable models available for significantly less money). The tablet is in much the same boat; depending on what sort of "art" we're talking about¹, this is likely the single most critical tool your artist will be using (note: I have some experience here).
Depending on the artist, they may or may not want some form of tablet/screen combination. Even though that could get quite pricey, my inclination would still be to go with whatever they are most comfortable using. (The flip side is if they're just trying to leverage you to get "shiny hardware". You could guard against this by asking if they have prior experience with the hardware they are asking for, or, as others have suggested, just give them a budget and ask if it's possible for them to work with that. BTW, this is basically how my own employer works; we get so much $$ to buy hardware and can more or less spend it however we think is most appropriate.)
(¹ Painting of any sort is best done with a tablet. Someone doing level design might not need a tablet, and a modeler may or may not, depending if they also do texturing or if the "modeling" involves "sculpting"/"3D painting". However, any sort of raster graphics is likely to benefit from a good tablet, and if they're doing something whose analog equivalent would involve hand-held tools, a tablet is almost certainly going to be worth it.)
As noted... the real question should be about the laptop. If they don't need portability, a desktop is going to be a better value. That said, depending on what they're doing, art work is pretty good at hogging resources, so it's not unreasonable to want a respectable system.