Driving a 4K display at 60Hz with rMBP I've been Googling so much my eyes hurt.

I have a 2014 rMBP.

I want to use it with a 4K display or UHD TV at 60Hz that's at least 30" big.

From my research, this means either using something with a DisplayPort input, or with HDMI 2.0.

The MBP only has HDMI 1.4 (I think) and is thus limited to 30Hz.

There are no >28" 4K monitors for less than $3,000.

But what if I used one of these DP-to-HDMI2 adapters? Would that make it possible to use a UHD TV with HDMI2 like this one?

Has anyone tried this kind of set up? Are there any decent 4K TVs/monitors for less than $1,500?
Your question has two parts: whether an rMBP can drive a 4K display, and whether any exist within your price range.

Apple's support site lists compatibility between their laptops and 4K displays.

If your Retina Macbook Pro is one of the 15" models, it can drive 60Hz MST displays at 60Hz through the thunderbolt port.

If it is a 13" model, it cannot - you are limited to 30hz regardless of output.

If your laptop does support it, the Asus PQ321Q is a 32" monitor that accepts a 60Hz image via MST. It is $2000.

There are also numerous 28" models that support 60Hz over DisplayPort 1.2. One example would be the $500 Samsung U28D590D.