other than the obvious spray bomb engine rebuild, check for rust under the; headlite brows, cab corners (front and back), floor boards, rockers, etc... it does appear that he has done enough work to make the cab acceptable, see if it's filler, metal work or just covering up surface rust. serious rust around the drip rail and roof area would be a deal killer for me.
you guys in texas have the advantage of surface rust only. i'm in the rust belt, we get the serious cancer. every patch panel or replacement part is available for the short box 3100 trucks. you could build a new truck from parts catalogs...
58 is the sexiest truck of the task force (55-59) series (though others may argue this point). not your first hot rod so decide how far you want to go.
as for the price: they don't make them anymore but they are available. price looks alright for the condition.