As said above, brake bleeding on a 109 is notorious for being done badly, some garages can't even manage it! The problem lays within the design, having two wheel cylinders on the front brakes rather than just one makes for more difficult bleeding of the whole system. There is a method to doing it right, have seen it posted before (either here or Series2club).
Gearboxes come and go quite frequently, keep an eye out for people breaking vehicles in the for sale section of this forum (and others) also keep an eye out in the 4 LR mags - £40-50?
Overdrive, you're looking at an average price of £250 (secondhand of course, or approx £900 new), and condition is never guaranteed, especially from online auctions with no viewing - ALWAYS view something you can't afford to be kna''ered.