Re: microwaves
fitting a fan to a microwave will never cause any leakage. the fan is outside the microwaves protected behind a wall full of holes. you should always check this wall is intact and no rusting has occurred to make the holes bigger and allow leakage.
apart from that its a question of taking it apart fitting the fan and re-assembly.
do quite a few microwaves and the bosh ones are great as the microwave section is Panasonic built on the later ones which means you can pay Panasonic prices for parts not Bosch. just did one the other day and did complete repair for less than the parts from Bosch
Dave