Yep, a decent upgrade it will most certainly be. Just try to find one that hasn't been heavily mined on. Ask the seller if its been mined on, and how hard and in what environment and how long. If he has good feedback he'll be honest.
waiting is an option. Though I wouldnt call a slightly older dx 11.2 gpu old tech. im also considering gtx 680 but it seems to be a bit slower than 280x and cost a bit more.
Oh that's an interesting price point. From team green there should be new mid-lower end gpus coming soon, though no concrete date but it would seem they should offer great performance if top end Maxwell is anything to go by. Myself I've seen several used Gtx 670s locally for $150 and 680s under $200 often. With those you won't have to worry about mining & the 256Bit bus Kepler cards can properly utilize all of their 2Gb Vram.
As for Amd, many cards will be Mining used but maybe ask and see what kinda of answers you get. Some people may be honest about how long etc, maybe come with a receipt for proof.