More than a few people have had the idea you have for making a pinball machine - google "homemade pinball machine" and start browsing. There's several projects out there. You might find it easier and cheaper to get an old beat up table and restore it, or custom outfit it with your own controls (do some research on the table before buying it - what you don't want to do is pay too much for junk, or go into a complete redo on a table that if restored could be a collectors item).
Something else you might want to look into just for giggles - see if you can find an old "Tomy Atomic Pinball" - this was a toy machine from the 1980s (or thereabouts) that was actually a pretty nice but small tabletop board for what it was. The amazing thing about it: It was completely mechanical (well, except for a motor and some sound effects). Awesome design worth looking into (note, though - that since it is a vintage 80s Tomy toy - it does have some collectors value - so keep that in mind).