If you install an earlier version it will give you partial support, then download the updated drivers through Apple Software Update. Windows 10 has support for every driver in a 2014 iMac as the hardware is pretty standard (custom drivers are made to reduce heat as well as to improve performance for Mac OS). If you don't feel confident in my post, you can always open up bootcamp on another Mac and grab the drivers from there.
In the case of Apple's drivers through ASU, they come from the same place that bootcamp grabs them, Apple does have a habit of waiting until the next release before publishing the current bootcamp driver to their website.
I know this because I've had to do this before. I downloaded 5.0 but needed 5.2 drivers for whatever reason, probably Windows 8.1.
Last thing, vladobizik, if you ever ran a netstat while downloading updates from Apple, you'd notice that they are downloaded from the same IP addresses (multiple servers handling failover and load balancing), the difference between the bootcamp download and the web page is one hides the ftp connection and automatically moves the software to proper location for install. I used to work in cloud computing and dealt with websites quite a bit, I'm all too familiar with these tricks. As a software developer now, I can tell you that if he downloads an older driver the only problem he might have with Windows 10 is hybrid graphics (in the high end MBP) due to different driver requirements. Since he uses an iMac the worst possible case for him is that Windows doesn't recognize the video driver and uses a generic driver until he updates it through ASU.