John,
I've had some hardtop experience. I had a car built around a Fibersteel Glasspar hardtop. It was gorgeous, and I don't regret having it for a minute. It was not weather tight, by any sujective standard.
If you need a drop-top to be rain-tight to justify it, you need to buy a nice used Z3, or Boxter. You'll spend the rest of your life trying to seal up a speedster from the wind and/or rain leaks that you will always have- hardtop or no. I have made it my "white whale" to get a speedster to the point where I can drive it in bad weather- and like Ahab, I may (?) have passed the point of obsession.
Pan-based speedsters (and I include any speedster that has a pan-replacement tube frame) typically have many different places where air and water can infiltrate the cabin, besides the windshield. I removed the carpet from the rear of my last car, and ran two full tubes of caulk into several joints to slow the air infiltration to a gentle roar. This does not indicate any lack of quality on the part of one builder or another- it is the nature of the "kit-car" DNA most of our replicas posess.
If you want a daily driver 356 replica- get out your check-book. Intermeccanica's roadster, and SAW's cab come the closest. Speedsters will always require compromise- and will amount to a battle between you and the elements, even when you spring the big bucks for a gasoline heater like some hick from Illlinois who shall go nameless.
I'm the kind of guy who doesn't accept conventional wisdom easily- I always think I can master the situation, given enough perserverance. I have finally, after two cars and tens of thousands being spent on a third, come to the conclusion that I can make a dent in the nature of a speedster, but not change it. I've accepted the limitations, reluctantly. They are not all weather, daily driver automobiles in the same way anything manufactured in the last 50 years is.
A boxter is a nice car- it'll be cheaper, and less frustrating for you to own one. If you can do without a speedster, walk away now. If not, be forwarned- it'll not be weather tight. No matter what.