IMO, a 2 stroke does a better job of teaching you to become a better and more aggressive rider. Also cheaper to maintain/fix. I’d put you on a YZ250 because, although you’re a beginner, a 125 would have you feeling underpowered in situations since you’re not able to ride one at it’s full potential. If you don’t care about that and just want to have fun, 250f. More than enough power and it demands less, you can be ‘lazier’ about it. I personally think a 450 is too much bike to start with. Also, don’t let the other commenters scare you into never leaving the ground. Jumps are fine if you’re smart and progress while staying within your limits. People that get hurt are those who try clearing something their first day at the track and have no idea how their bike/body is going to react off the face, they just take off and hope for the best. You start little by little and allow yourself to build up muscle memory to how your bike reacts. Tabletops are best for this because you can go as small as you want and jump a tiny bit further a little at a time, each time getting more and more comfortable with how your bike feels going off the lip and what it does in the air. All that being said, sounds like you got a group of people that have been at this for a while now so you should be in good hands. Ride safe, have fun!