Any and all answers are welcome thanks!
Remember, a contract may help you during a legal proceeding. But that's the worst case scenario - the best case is to preempt any legal action in the first place by clearly communicating and by providing great service. A contract is an instrument used to resolve disputes - it's best not to get into a dispute in the fist place. To truly protect and your business, consider purchasing general/professional liability insurance.
Invariably, clients are going to want changes. And that's a good thing. It can mean more business and more profit. You need to determine if these changes are in scope or not. Don't be afraid to ask for more money and revise your original estimate when you encounter feature creep. But you have to be able to clearly explain and justify any price expansion.
Building on that - don't be afraid to walk away. If a client isn't willing to pay more for more features, find a way to make a graceful exit.
Long-term support can be another revenue stream. And as the creator of the original product you are in the best position to provide quality, knowledgeable support. That's valuable and worth money. Make sure to communicate the long-term support strategy at the beginning of the project, be it hourly billing for bugfixes & patches, or a monthly/quarterly/annual payment for ongoing maintenance. Your original estimate should also budget for initial bugfixes and tweaks near or after launch.
New site - a basic proposal, usually about a page or so in length. A sitemap, maybe basic user stories to describe interactive functionality. If it's a complex site, charge for discovery/planning as part of the project and document enough to create a reasonable scope.
Modifications - Clients will always, always have suggestions and changes along the way. Buffer your bids to expect this (~20%) and don't be afraid to issue change orders (short written descriptions of new functionality) with appropriate charges for major scope creep. Managing expectations is key.
Post-launch support - Yes, you'll often make more money this way. Hosting can be one way, for clients with lots of regular changes try to get them onto a retainer where they buy a block of hours at a reduced rate each month, provides cashflow and steady income for you.