Think there could be lots to look out for, dents in frame, check for hairline cracks. Also dirty (not dusty) roadbikes more than likely means the previous owner didnt take care of it. You cycle, so I guess the basics remain the same, perhaps MTB's have more scrathes and so on but you'll quickly see if its a dodgy bike or not. Are u buying "used"? Ask to test ride it. When wife and I started road cycling we bought used "cheapies" and the addiction took off from there. Bike size...see posts in this topic for your answers. Good luck.
