I took a similar route but instead bought a late 2014 MacBook Pro loaded with 18 months of AppleCare left.
The good part is your previous Mac lasted 7 years on 8GB of RAM. So if you double up the RAM and now have a current generation processor which is waaaay better than your 7 year old processor you should be extremely happy. Also you can't upgrade a SurfaceBook or most of the "UltraBooks" out there so I would start to accept this as the norm, not the exception.
The 4K resolution and TouchBar will be great in a couple of years but like Touch on a SurfaceBook its mostly gimmicky. The key point is that Windows 10 sucks and I would rather have a non-upgradable MacBook Pro running OS X than a buggy platform like Windows. Also the integration with the iDevices and AppleTV & Watch you have will be a much better experience than a Windows/Android or if you could find one, a Windows/WindowsMobile combo.
Time will tell whether the TouchBar will be awesome but for now I was willing to save about $800 and get a used device.