If you're not concerned with aesthetics, sand it as much as you can (the more the better), I would use 220 and then 600 because you're going to want some smoothness.
Then use one-step primer, this stuff can actually be sprayed directly onto rust and reacts with it chemically. It's better to sand the rust off but you don't have to get it all off. You do need to expose all the rust, so I would make sure to remove all the paint when sanding. It comes out black once it's done so you're going to have black, they don't have other colors as black is a byproduct of the reaction.
You've then gotta paint the primer with either BC/CC or a single-stage automotive grade paint. I think the instructions on the primer say you can leave it without painting but I would paint it since you're not going to want to be doing this again. If your not worried about aesthetics just paint the black, if you want the color right, prime the black with a regular grey our white primer and then paint that.
I wouldn't use a bedliner like Line-X. It's the wrong product for the job and you'll have problems.
So, person one is sort of right.
Person 2 is correct if you're not using the materials I mention above.
Person 3 is right if you're using something like a bedliner or have absolutely no idea what you're doing with paint.
This can all be done in a home garage with aerosol cans in about a full day, it won't look pretty but you can get the product if you're thorough.
Places you can fail:
1) You don't sand of all the paint and there's rust underneath the paint that the primer doesn't get to that then continues to grow.
2) You paint the primer before it's totally done reacting and you get bubbling and reactions that result in the paint not laying down right. Check the can for instructions on how long you should wait)
3) You don't wipe down the surface before priming with a grease and wax remover, mineral spirits, etc.
DISCLAIMER: THIS WILL NOT LOOK PRETTY, IF YOU WANT A PERFECT JOB, GO TO A BODY SHOP AND GET THEM TO PAINT THE ROOF, THEY'LL PROBABLY CHARGE YOU 100-200 BUCKS.