For some perspective, 35mm photo stock (135 film) features frames of 36mm wide x 24mm tall in landscape orientation. Yes, photo types will give the measurement in portrait, HxW as 24x36mm. To compare scanning to digital stills you might get from your current digicam...

The absolute highest resolutions you can expect:

3000 ppi ~ 4252 x 2834 = 12 MP
2000 ppi ~ 2834 x 1889 = 5.3 MP

These numbers are usuing the full frame and don't discount any pixels for crop to eliminate edge shadows and other peripheral artifacts or slide borders.

The amount of grain and other "deficiencies" you pick up will greatly depend on the film stock, focus and exposure settings used when the images were taken. Not to mention possible degradation and abuse of the negatives and/or slides over the years.

I think I still have negatives for everything I've ever shot on film, but I know for a fact my parents have very few negatives for their oldest prints. This might be something worth pooling up content within the family and then getting it all done at once for a bit of a discount.