Since your son lives in his own home at college, you will not have to add them to your policy if they drive your car less than ten times per year, with most companies. Some insurance companies offer a discount if you add a college student who lives away from home and has limited access to the insured car and seldom drives it. Every company is different so I would check with your company for their specific rules.
I recommend setting up your insurance policy before you pick it up because you will need to have insurance as soon as you drive off the lot. The seller shouldn't have to be involved when you're choosing a policy. Every state has minimum liability insurance coverage requirements which are included on every policy.
When adding another vehicle to the policy you would also add the driver of the vehicle to the policy. Generally, insurance companies will not allow a person not living in the home to be on the same policy.
There are too many unique rating factors that determine your car insurance rate for me to give you an exact answer. Based on your location, the average rate of car insurance in Colorado in 2017 was $1,435.