8 speed STI will work perfectly, so long as the shifters aren't Dura-Ace, since 6-8 speed Dura-Ace rear derailleurs had a unique actuation ratio that only DA levers work with. That caveat aside, the 6-speed 105 will index just as well with an 8 or 9 speed STI shifter as it did with the 6 speed downtube levers it came with.
Front derailleurs can be trickier, so just get one that came from an STI group. The 9-speed 105 is a great choice here... dirt cheap, pretty, shifts great, and about as reliable as a shovel. Actually, that's true of 105 parts in general, and has been since 105 made the "group bike" a thing back in 1987.
--Shannon