I’m 5’11”, ~180-200 lbs (it varies) and I started on a Honda Nighthawk 250 which has the same air-cooled 234 c.c. parallel twin-cylinder engine as the Honda Rebel. The Nighthawk was plenty powerful enough for a beginner and would happily cruise all day at an indicated 65 mph. It would slow down a bit (to ~55 mph) going up long hills, but kicking it down a gear would soon get me back up to 65. It *could* do ~80 mph flat-out going down hill with a wind behind you, but was not really comfortable at that speed due to vibration, etc. I used to take the Wife as pillion (passenger) occasionally too (she’s quite a bit lighter than me) and it carried us both around town with no difficulty.
The Rebel might be a little more cramped than the Nighthawk 250 (I’ve never ridden one), but as Elwood says, “at $800 you can’t lose as a trainer”; ‘seems like a good price to me. If it’s in good condition and runs well, you’ll easily sell it for $800 in 6 months or so I would think; when you’re ready to move-up.