1) Is Insurance Doctor a good company?
Insurance Doctor is obviously an independent agent or broker representing several insurance companies.
Their website has links to local reviews:
You can also check them out on review sites like Yelp:
You can contact your state's insurance department to see if any complaints have been made.
You can also ask the agent the name of the insurance company that will be writing the policy they are quoting.
2) Is it worth investing $650 for insurance per year to get maybe 50% of the market value of my car in case its totaled again?
Only you can decide that after asking yourself do you have enough money in the bank ($5000 to $10,000 that you'll never touch) separate from your income and living expenses to instantly replace your car if it gets totaled or stolen and nobody pays you for it?
If you can truthfully say yes to that question then you can save yourself a lot of money if nothing happens to your car between now and when you are ready to sell it some years in the future.
I've had my 08 Crown Vic for 4 and a half years now. I can replace it for about $3500. I have that money stashed away and have probably saved at least $2000 by having just liability insurance for the past 4 and a half years.
PS: I suggest you get quotes for a complete policy from other direct writers and through other independent agents.
A lot of insurance companies will insure rebuilt vehicles for comp and collision along with the liability coverage.