What is Windows Phone equivalent to Google Nexus series?
The closest equivalent to the Nexus series in the Windows Phone market would be the Lumia family, insofar that it's also (as of mid-2014) being produced and supported by the same company that makes the OS - Microsoft. Luckily for you, it also offers the largest selection of devices of any Windows Phone manufacturer by a considerable margin (so large that you may need help with deciphering the naming system), as well as (arguably) the best hardware. So in that respect you're spoiled for choice.
devices receive updates directly from Google, without having to wait for manufacturers or carriers to push them to me
The Windows Phone Preview for Developers programme (in the process of being rebranded as Windows Insider) that you mentioned offers precisely that. By signing up you'll be able to receive new versions of the OS as soon as they're released, without having to wait after the carrier or manufacturer (although see here for a caveat or two). The Preview programme is available for all Windows Phone devices (except those that originally shipped with Windows Phone 7).
the OS is not customized (stock)
Windows Phone devices cannot really be customised in the same way Android devices frequently are, with the manufacturer adding resource-heavy skins on top of the stock UI. Any Windows Phone device will offer the same base experience. Where manufacturers differentiate their products is with additional apps, but all of those can be uninstalled if you prefer a completely stock experience. Still, I don't think you need to be concerned about this.
updates are still available for a long time after the release of the device
In terms of longevity, pretty much all WP8 Lumia devices released so far (starting in September 2012 with the 820 and 920) either have already, or will soon, receive the latest version of Windows Phone 8.1, as well as the Lumia-specific firmware update named Denim. Also, most, if not all of these devices should be able to upgrade to Windows 10 when it's released later this year. However, if you want to play it really safe, you may want to rule out devices with only 512 MB of RAM or less than 8 GB of internal storage.
It's hard (not to mention off-topic for this site) to make a specific device recommendation. If you want to get by on a budget, a previous-gen flagship like the 920 or 925 is still a solid choice. But if you're at all interested in taking advantage of newer features like SensorCore or Miracast, I'd suggest a higher-end third-gen device like the 830.