That big DeWalt will quickly tire your arms out. Use the 5" Random Orbit - the DeWalt I have is a good one. I've used it all day for days on end and it keeps on ticking. Buy your disks by the box - the copper in bottom paint wears them down quickly so you'll need lots.
To fair things up after the coarse prep is done, use one of the boards like in the pictures - they are for autobody work and they really "flatten" or fair things up well. They use pre-made paper strips 18" long. You push them on the diagonal - big X motions. Next to one of those plywood 2 or 3 man longboards, they are the best fairing tool around.
Unless you are very near the gelcoat, remember to start with coarse paper - I start with 40 grit on the orbital and usually 60 when I get to the board. I find people are often afraid of using the coarse paper and start with 80 but it just wastes time & paper. For most of the "finished' bottoms I've see, they could have done better if they had ONLY used 40 grit.
I've tried the drywall screen sander thing - forget it. They are designed for sanding drywall mud which is extremely soft & easy to sand. I found them useless on a boats bottom.