My little take on this topic:
There was a time when upgrading to Medium Format was evident to gain IQ. People started by using 35mm, with a long learning curve and jumped to MF/LF, to pursue the goal of high quality photography. In film days, the gap was very visible, it was logic.
Jumping to MF was kind of mandatory to "future proof" your hard work, even at the start of digital photography. Then the 35mm industry, with the Cmos technology, started to up the pixels count on their wafers. Just remember the Mamyia DF vs 1DsIII fight...
This is interesting to see that MF industry stay with CCD technology and do not jump into Cmos kindergarten fest. Many posts above show why and skin tones/wavelength interpretation seems to be the key. Leica M8 (or even the old digilux) output very pleasant skin tones, better than Nikon or Canon Cmos based DSLR.
The visual impact between CCD and Cmos should not be minimized, at all. Some had a look at SIGMA with the foveon because, even if it is a Cmos sensor, it output something close to CCD rendering as well as the D700 because of his big pixels. Some do not like the M240 output because (and it is understandable) of the lack of crispness or whatever, like if the M9 had a soul and not the M240. I, and I'm not alone, see what they mean.
Today, 35mm industry try all the possible tech ways to not die. Incredible amount of pixels, features and automatisms. D800/e, Sony A7r and so on ... Wonderful tool compared to ancient technology even very old MFDB.
So, as some photographers said to me one day, if you want to future proof your work/passion, do not jump on the first little digital MF you find on the market. This is a hard way because you will need to learn it hard and seek lenses + study all your graphic chain behind thus engaging more costs. If you jump into MF, buy at least IQ260 or H5D60 >> this is the step to go beyond (in the future) what latests 35mm DSLR offer and will offer soon. If you do not have it yet, you will need a good computer and a good screen + calibration tools (Mac + Eizo + Xrite). There is no little steps today; you move in or not... if you are pro or very passionate.
It is why "crop MFDB" like S2, pentax and all the entry level stuff (H4D40, P40) are a dead end by today standards. A D800e + very good lenses (Zeiss) do a very good job and come very close to the end result of those backs, period. This is a wonderful tool for the one who can't invest in bigger imagers.
And here come the softwares... With the actual plethora of pro-softwares you can find around, you can render a LOT of things, even shaping a 35mm shoot in a sort of MF looking shoot. That's the hard reality, truly. But it need some practice and it is time eater.
Now, if you really want to have a MF "look" and really want a MF, nothing can stop you apart yourself. You have the choice. If you come from a 5D or a D700 and want MF look, you can find (even new) the old and reliable Mamiya DF. ISO50/100 only, CCD look, not expensive even the lenses, possibility to use it like a tech cam with bellows and adapters ...
This is not a killer MF but it is simple to use. Sensor is 2x the size of 24x36 with same density as the D700. It is a double D700... but with CCD color rendition.
You also have the film way, who is not dead at all but take more time, for sure. Incredible DR, real rendering on a 6x7 sensor, less expensive on the run than a IQ260 system
So IMHO, for your little
budget, there is three pragmatic ways:
The first, illogical by today standards but yet pragmatic, is the Mamyia DF. 22MP, 8500€ brand new, inexpensive lenses, very good rendering, big sensor, CF and SD card, rugged ...
The second, the most pragmatic and future proof, is the D800e + plethora of good lenses (135f2 ZF2, 21f2.8 ZF2 or AF equivalents in Nikkor line-up, + screen/computer). If you do not like skin tones out of this camera, try to find someone who can teach you how to improve that in post quickly.
The third... the film way, the real MF. You can rent a Voigtländer Bessa III 667 (or 667W) or his Fuji equivalent and burn a roll or two with it. Scan it with an Epson V750 and see by yourself. You can buy a used Rolleiflex Hy6 Mod2, because you will have the possibility to plug a MFDB on it later. Absolutely awesome camera and lenses, very underestimated.
You have the choice. But for real MF venture, 15k$ is close to nothing (especially if you face some mechanical problems and need to pay the bills, this is why I don't speak about Hassy...).