The Air is a great device, but is clearly being phased out by Apple - the 11" now gone, the 13" can't be far behind (for price, Apple clearly wants users to buy iPad Pro, which is
not the same thing at all), so not sure its a great investment now. I have an 11" Air and it works well for the stuff I do, but no major software demands - some picture manipulation - but I've never tried AutoCad or similar on it. But now an 'old' device and the price leap from Air to anything else may mean this is my last Mac. But I digress...
Like I said, big leap in price then to MacBook / MacBook Pro. MacBook 'horsepower' has been dissed - some argue worse than Air - while the Pro line has faster processor, more RAM - which can only help with AutoCad. That said, I've heard reports that some pro users of Mac products (which would probably include users of AutoCad) are less than impressed with the bang for the buck of new Pro.
Meanwhile, whatever you decide to go with, its always worth keeping an eye on Apple's refurb store. Here they sell devices returned - buyer remorse, wrong model, something wrong - and they do a full rebuild and check and resell with usual retail warranty. AppleCare can be bought for them and often it includes free shipping. US version of the store here -
Refurbished MacBook - Apple