Hello George and welcome to the forum.
I have owned an S2a and several EC's. Don't worry about the mirror slap. It happens too late after the shutter to cause problems. Use any shutter speed you want while using a tripod. I don't know what you read but I have never had any problems or heard of anyone having any problems.
Early Bronica cameras, film backs and lenses used to show up often on ebay here in the United States. They have slowed but still show up occasionally. Most lenses are dirt cheap unless you are looking for a 40mm Nikkor or a lens longer than 200mm. Like Trask says, the 80mm Bronica is hard to find but you don't really need it since you have the excellent 75mm Nikkor. I also highly recommend the 150mm Bronica lens for portraiture.
With your S2a you can use any S series (S, S2, S2a) back. I can't remember for sure but I don't think the EC backs will work on the S series cameras.