I guess the hardest part would be sourcing the slate slabs. The actual table wouldn’t be so difficult, except that it should be built using a hardwood which can be expensive. I find that some projects, like those that have to do with water (plumbing, roofing) are best left to the pros.
Likewise, bar stools and pool tables include enough unique, hard to build or source parts that is warrants buying one from a manufacturer.
For example:
A pool table plan would require the slate slabs, felt, bumpers, inlay markers, corner nets, leveling feet that may all be very hard to find individually and you’d could (waste) spend more time searching for parts than playing pool on a good quality manufactured table.
See:
http://shrsl.com/?~4do for available pool tables – most with FREE SHIPPING! That’s usually the biggest expense!