MTB with gears or non gears, suspension or non-suspension for stunts?
If you want to learn BMX type tricks then rigid    


BMX bikes come rigid   
BMX bikes also come single speed  


Suspension and gears cost money. 
Put that money into better frame and components.  

In a mtn bike don't worry about dedicated single speed. 
A single speed conversion kit lets you go to gears later. 
Even if you don't want to convert it for yourself it will tend to have better resale  

Get mtn bike with a low slung frame and a tube that lets you go way down with seat
(some seat tubes have a bend). 

Consider a 26" used mtn bike with a nice frame that has a worn out shock and gears and convert it to rigid single speed. 
Since mtn bikers want 29" there are some really good deals on used 26" mtn bikes.
Especially 26" mtn bikes without disc brakes. 
You don't want to go so old that it has the 1" treaded fork. 
You want a 1-1/8˝ threadless steerers.