Buy a hardtail frame,no not a slalom frame.Not an XC frame.Pick up some fucked geometry freeride hardtail frame from a German company that has one big ass toptube as roughly half of the frame.
Pick yourself up a Monster T.Look really hard for the late model ones that weighed 14 pounds and had like 8'' of extra fork leg sticking out below the axle.Skip out on buying the older ones that were 8 pounds,worked amazing,and felt fucking good.
24'' wheels.A must have.
Keep this thing versatile.You want the ability to go uphill and downhill.Oh fuck,how are you gonna shift?How's your chain gonna stay on?Answer?...Hammerschmidt.
Go to the local shop and pick up some pedals they took off of their rental bikes.Cheap,metal,and hella grip.While you're there ask if they have any rear reflectors,because the last thing you want is to get hit by a car after hucking that 12 stair to flat.
You've got yourself one baaaaaaaad motherfucker.