There are loads of smaller cruisers than the S50. If you’re taking the MSF course chances are you are/will be riding a 250cc cruiser. I wouldn’t worry too much about getting a bike you can grow in to. You can pick up a used Rebel or Virago 250 for $2K or less and you will probably get most or all of that back when you sell it to move up to a bigger bike.
As far as who to buy from I would check to see if the msf course provider has any training bike for sale. That’s where I got mine. The mileage was really low, it had been well maintained (they gave me all the service records) and although it had been dropped a few times it had never been ridden on the street and probably never over 25 mph. It’s like buying a bike from the proverbial little old lady from Pasedena.