Any twin plate will rattle, that's the way it is with the design. Any lightened flywheel will also cause a rattle to some extent. A lightened flywheel isn't going to decrease the lifespan of your gearbox any more than running an extra 200~bhp through it already is to be honest. It's designed for 300 and running more will be putting more stress on it. We all know the Getrag is pretty robust so really don't worry about it.
The OSG is a very good bit of kit, I know a few people who swear by them and I think you'll find Chris speaks very highly of them, which is enough of a recommendation for a lot of people.
I got that price for the rebuild kit direct from a Jap supplier website, I think it might have been Greenline? They do two levels of rebuild, one is just friction plates I believe and one includes the cover plate as well.