I don't know that I've ever seen mention of a 'minimum' speed rating for SVX tires. But, given the size, I think you're going to be hard-pressed to find a cheap tire *not* rated above 100mph.
I have Dunlop M+S rated all seasons on my SVX and even they're rated to 130mph.
I've never bought tires at Costco, but it sounds to me like they're full of it and just want to sell you more expensive tires. I had a Costco membership when I lived in VT, and their tire prices were no better than Sears. Do yourself a favor and get a Sam's Club membership instead.
I might be wrong, but 'we won't fit anything rated below <insert speed rating> for that car' sounds like a bunch of horse pucky to me.
Edit: Looking at Tire Rack, I think that the closest thing to an RE75 is the RE92, which is H- (130mph) or V-rated (149mph). If you're looking for a guideline, I guess there it is...