My new to me Nishiki Colorado

My new to me Nishiki Colorado

Hello all, i’m more of a car enthusiast then a bike enthusiast but have been wanting to get back into mountain biking for a while and was hoping to get some help. Picked up this Nishiki Colorado for $30 and i’m hoping to fix it up.

The goal to make it decent is fix the brakes, fix the chain, get a new seat and seat post, and get new handle bars to work for my height. Anyone know where I could find the size of the seat post as the current one is stuck on and actually needs blunt force to get it out as it’s too big. Thanks in advance!