Welding sheet metal is an art! One second too much and you have a hole twice as big to fill. Also easy to warp a panel with too much heat. Small little quick tacks with the mig welder will teach you a whole new meaning to taking it slow! Little dings aren't too bad, but 50 or a hundred in one panel can make you nuts getting an even finish for painting which is where I am at now. There are some good hammer and dolly sets for relatively cheap prices, that you can get a Harbor Freight, etc..., that you'd be surprised how quick you can pound back dents and dings with. I bought a book to follow on the steps to paint and car and one on restoring old trucks. There was some useful information, but ultimately I found most of the references to products, etc., out of date. Google searches on specific subjects will get you a wealth of info. There are some great youtube videos on welding sheet metal and body work, as well as paint spraying techniques. There are people here versed in all of the above, which is also helpful.
There are some great threads here on car builds, that follow logical sequences to building a car. Don's project 3k thread is one worth reading from start to finish and has been very helpful to me. My thread on the other hand is a great example of bumbling my way through my first project, getting distracted along the way! My approach way collecting parts I needed for the engine rebuild, while doing body work, rust removal, etc.
If you have the space and tools and a helper, taking the body off the frame and starting on the frame seems the way to go. Hope some of this is helpful,
and what Pat and the others have already said is very important, be mindful of what you may be inhaling, some of the paints and chemical as well as sanding dust can mess your lungs, eyes and skin up in a hurry.