I find the Tallboy seat great - wider, higher and set back more than standard.
Makes the mid-pegs less cramped and causes you to sit more upright, so far better than the standard seat for me at 6ft 1inch tall. Re the engine, you may find it useful to compare the 107 and 114 motors.
The rear brake on mine is crap, dealer bled on first service with improvement, but any adjustment of the pedal to make it more vertical will give more leverage and seemingly more effect.
With lack of history and concerns about rattles I would leave it, get one where you don't have th3se worries, plenty more out there.