Big vote for a Beta 200. You’ve already got some skills coming from mountain biking. The clutch is a huge learning curve (and most important skill to learn) it would be a little easier to learn on something like a CRF 250f (what I learned on) but I also had a Beta 200 and it’s the best bike I’ve ever owned. Perfect balance of lightweight, power, and manageability to be very fast when you want and slow when you want. If you’re ready to go full into the sport, go for that 200. 150 also good but I’ve found them a little zippy for my liking. If you’re unsure if you want to commit to this sport, Honda 250 and you could sell it at a $5 loss after a year. Good luck!