I wouldn’t recommend the s50 for you, maybe the s40 though. The s50 is nearly 100lbs heavier than the s40 which is quite a lot when you are just learning how to ride, plus the added power can get a new rider into trouble quicker. Even the s40 is a pretty dang big bike compared to the vstar 250 or the rebel 250.
When it comes to sportbikes vs cruisers the seating position varies quite a bit. Almost all sportbikes are going to have you bent over more than a cruiser would, but some bikes (like the gs500 you mentioned) have a much more upright seating position. I personally started on a gs500 and it was a great bike. Haha, honestly it was probably too big for me to start out on, but I really loved it so I got it anyway.
I would look for used bikes off craigslist, and if you can try and find someone who knows something about motorcycles to go with you when you check it out. I met a guy off a forum who helped me pick up my first bike, and he even test rode it for me too! Where do you live at?
Have you checked out the vulcan 500? That is a great looking bike as well if you are swinging more towards the cruiser side of things. Hope that helped, let us know if you have some more questions.
~Best Beginner Motorcycles Admin