What bike is good to practice stunts?

A closed parking lot is asking to get you in trouble with the law just because of liability for the property owners. If you are serious and not just a poser, then you would use a dirt bike, (gasp) on dirt. Use the dirt bike for riding on dirt. it's what they are meant to do and the street is absolutely not appropriate for that. A 750 is a massive step up, and you didn't mention whether it was a street bike or dirt bike so we are left guessing. A stunt fail in a parking lot will be a good bit worse than a stunt fail on dirt. do some diggin around and see if there are any local groups that are into dirt riding. you never know what will turn up.

What kind of stunts? BMX bikes weigh 30 pounds and you can ride them vert, dirt bikes go off massive jumps, and you can do wheelies and stoppies on street bikes. I can see how bmx and MX are related but where does the 750 come into the equation. I would keep your riding on the dirt, its more fun and less illegal.

Stick with the 125 unless you have a close relationship with a bodywork supplier and the friendship of a bearing supplier, you will be seeing a lot more of them. A dirtbike is made to take the kind of abuse that you will be handing it. And you are wrong about the mechanics the suspension set-up and weight make a significant difference.