If you get someone that's breaking a GT/Glanza your prob looking at a few hundred quid max for all the bits. I went all out a sorced a 6speed out of a corolla g6 which made it a bit more expensive but it was well worth it.
6th is the same as 5th in the 5speed at 60mph you just get there quicker. it takes load off the engine the only reason why my 6speed broke was because the end of the drive shaft broke off into the box an chewed up the plannet gears. also 1st gear is so short its kind of pointless 3rd is where the 6speed shines.