Removing ALL of the rusty metal and fitting new metal is really the only way to go. When a pre formed panel is available it's often quicker and neater to cut out more metal so that the whole repair panel can be used. If you can't weld and have no equipment calling in a mobile welder will be cheaper than a body shop if you do the cutting out and mocking up first. A modestly priced welder, not necessarily the cheapest from Harbour Freight, check Craigslist, would allow you to DIY. Floors are a good place to learn, as perfection isn't a major criteria here. Don't try to weld rusty metal though.
Check out
http://barzettiswelding.com/ They do mobile but they also do short courses that start at 1 hour.