Well, I don't have any experience with Intel Iris Pro 5200 Graphics which MacBook Pro 15" 2015 have onboard, so I cannot say it will operate good enough with AutoCAD.

I have MacBook Pro 13" 2015 with Intel Iris 6100 GPU. It gives me decent performance in most cases with average drawings. But if i connect it to the external 4K monitor, I'm starting to see some lags inside AutoCAD and in macOS UI, especially if I gave a lot of applications running simultaneously.

You also can look at the discussion about Iris 6100 vs Iris Pro 5200 here:
https://forums.macrumors.com/threads/intel-iris-pro-5200-vs-intel-iris-graphic-6100.1855181/

On the other hand I'm not sure buying MacBook Pro 15" 2015 with integrated GPU will be the "correct purchase for the next 3 years at least". Who knows what can happen in a year or two with the software requirements (for AutoCAD and for other applications), believe me - your desires on use of the computer won't be limited to AutoCAD only.

Maxim