Finding a shopping cart will be the easyest part. Show me your frame plans and I will add steering to them. I dont recommend using scooter brakes on a Kart, they will heat up and be destroyed after their first use. Use hydraulic disk brakes off a motorcycle, the motor needs to be mounted on a flat piece of steel or somthing stable. You sound like you just want to hack up a scooter and glue it to a shopping cart, cheap......yes, relieable? probably not. I have made a few go carts so I have a good idea what needs to go where. Show me pics of what you plan on using and I will do my best to make it work for you. Are you in CA?