Its always nice to hear about a new rider and his or her enjoyment of the experience of motorized two wheel travel. And I understand the need to take it slowly regarding riding on the Interstate. Although it is probably the safest place you will ever ride (no cross traffic, limited entry and exit points, two or more lanes of well maintained road, and greater bike stability at Interstate Highway speeds), it can take some time to get comfortable at the speed you will be riding at. But after awhile you may find that you will love being on the Interstate, a place where you might be riding at 75 or 80 mph without a thought or concern.
As to what bike is best, that is certainly a subjective subject. Even for the same person, the "perfect" bike will vary as the experience and needs levels change for the rider, and even as the time of day changes. Most of my 35 years of riding I have had a single bike. I started with a 150cc scooter, went on to a small cruiser (700cc) and then over the years have had sport touring bikes, bigger cruisers, and now a full dress touring bike (Honda Goldwing). There is no way I could have gone to the Goldwing early in my riding experience. It took awhile to work up to where I am fully comfortable now on a 900 plus pound bike, even as I am entering my senior years at 76. And while the Goldwing seems pretty "perfect" for me now, I am now the owner of a second bike, a 150cc scooter (returning to my beginning). For errands around town, nothing beats this little scooter with its underseat locked storage and pure fun to ride. Getting out on the road is where the Goldwing excels. So you may find that you will keep changing what for you is "perfect" or may be like many riders and end up with several bikes in your garage for different needs and wants.
As to a windscreen, I personally think that this is absolutely necessary if you are going to ride at highway like speeds. Not only the buffeting issue, but the protection from flying debris, bugs, etc. a good windscreen is critical. I had a Triumph Thunderbird (cruiser) that had a detachable windscreen, that came off or on in seconds. I used to leave it off if I were cruising around town, rarely going faster than about 55, and put it on when I knew I would be riding at Interstate Highway speeds of 70 or more. I'm now thinking of adding a windscreen on my scooter, since I find that at about 50 mph the wind blast is sometimes uncomfortable. I suggest looking at what National Cycle has to offer for your bike since they have a pretty broad selection of windscreens at reasonable prices.