I think Louis said it well; the DX is basically a sportier version of the Big Red's you are used to.
As for things to look for,
I would check the front shocks, pus on the front end to make sure they still compress and rebound and are not bent.
Check the electric starter, the starter motor tends to be a weak spot and may need to be rebuilt.
Clean rear end gear oil is always helpful for keeping the rear end working.
All in all they are good machines and if it runs well and looks ok then it is worth the $400, you just have to decide if it would fit your style of riding.
1984 Yamaha YT 60 Tri Zinger(current project)
1983 Yamaha YTM 200 (rear suspension kit)..... long term project
2004 BMW R1150R
1972 Honda CT 70
2007 Lifan 110