Depends on what you consider "good" and "affordable".
My friend has a cheap Samsung HDR TV around $500. Yes, it technically does HDR but it's edge-lit and the peak brightness is around 300 nits, which is shit. HDR content does not look noticeably different on it compared to others.
You're looking around $1000 for mid-range HDR TV's right now.
In my opinion, if you can wait a few months with your current TV, see what is out there on Black Friday and if you can't get anything just get the Pro. It will benefit you on your current set already. Then, next year with the improved HDR sets you'll get a better bang for your buck and the tech in this year's $1k mid-range may be like $700 next year. I think with both Sony and MS pushing HDR for gaming that the manufacturers are going to take that into account better for their 2017 models.
Yes, display manufacturers are only including it in 4k displays. Not for a technical reason, but it's easier for the display manufacturers to sell you all this new tech at once than to have "HDR", "WCG" and 4k be three new, different technologies consumers would be able to choose from. Thus, it's all a part of the "Ultra HD" label.
"Ultra HD Premium" is the seal of approval that it's a good TV and meets certain quality criteria on resolution, HDR and color reproduction. It's relatively new (from January I believe) but you'll see it more in 2017 TV models. I would _highly recommend_ only looking at TV's with that logo. If it doesn't have it, there's almost certainly some sort of catch. The only thing on top of that to look at is input lag while in HDR mode. If it has the UHD Premium logo and it has low input lag, you found a good set.