Motobecane is making some aluminum frame bikes that have specs and parts equal to some more expensive bikes. I think you'd really enjoy riding something lighter in weight after riding a K-Mart bike.
There are some great deals online if you can do the assembly / adjustment. You may still come...
Something's wrong. You won't run out of coolant like that unless there's a leak, your head gasket is bad or maybe a bad thermostat is causing overheating and your coolant to boil off.
I would pay to find out what's wrong immediately. It could be serious enough for you to blow your motor driving...