Welcome, For the flatbed it all depends on budget and steel is cheaper if you can go to a steel yard and pick it up. With proper prep and finish should last. The one thing is if you use too much steel and go for a fancy flat bed with lots of steel it will slow down the truck and lower mpg etc. Now using aluminum is cheaper on the shifting but welding it can be a PIA with tig for that project so spool gun would be a must. For the deck, I would use diamond tread and put a rubberized coating on it for traction in wet and protection from dumping stuff on the bed. (pictures are not mine just example) this is the flatbed I would emulate for a lighter weight flatbed. then I would get a box like the one below to get more storage and bolt it right to the bed.
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