For such a large area, you’ll need a better sander. You could do it by hand, but that will get tiresome, because to paint a car you have to sand a LOT. And it’s not even the prep stage that should be your concern with respect to sanding. You can take the paint off with anything you’ve got. It’s the sanding each coat of primer and paint you lay down that gets to be difficult.
Painting a panel from an aerosol can is tricky to get good results, but by no means impossible. A body shop owner I knew one told me you could paint a car witha brush and it would look good if you sand it right.
The basic idea is to sand and strip the old, loose paint until it’s as smooth as you can get it. Wetting the surface can help you spot irregularities. Put a couple coats of primer, letting each coat dry before wetsanding it. You spray, wait, and sand primer as many times as necessary to get a perfectly smooth surface. It should only take a couple coats.
Then, you apply color coats. Thin coats are what you’re looking for. Each coat gets to dry before being wetsanded with fine grit paper. You spray until the color is built up the way you want it. It will take a few coats. Assuming you have a paint that requires a clear coat, you give the final color coat a very fine wetsanding, then apply the clear. If you have a paint with no clear coat required, you sand the paint with progressively finer sandpaper, ending with something like a 1500 or 2000 grit before moving on to a polishing compound to make it shine.
You should use a tack cloth to remove dust just before each time you spray. Getting a dust nib in your pain job is fun for nobody.
In short, it will take you forever and be a lot of work if you want it to look good.