Many thanks,
Saab Dastard , those suggestions are one step beyond what I've been thinking of right now, but are certainly interesting. My current desktop PC (which I'm using right now) is a Chinese fanless i7-8550U box, with 16Gb and a 2TB SSD, running Win 7 Ultimate (hacked to allow it to run on an 8th gen processor). I have a decent Cherry keyboard and a 24" 1920x1200 monitor that are only about a year or so old, so not really up for replacement just yet. This PC works pretty well, and I'll probably keep using it for at least a year or so, not least because I use AutoCad a fair bit. I can see a time when it's just going to be easier if we switch entirely to Apple stuff, though, just because this machine is our only remaining Windows machine.
The i7-8550U is roughly twice as capable as the base model MacBook Air, too, and AutoCad is really snappy on it. That's probably the most demanding app I use, although some of the 3D rendering apps I use from time to time may well stress things a bit.
I had thought of switching the desktop to Mint, but AutoCad objects to running in a Windows VM for some reason, yet runs fine natively on Win 7.
Not 100% sure if I'll make the switch to an iMac as a desktop yet, my plan was to dip my toe in the Apple water with a MacBook, and then make a decision as to what to do with the desktop machine at a later date.
Thanks very much for your experience,
anson harris , they pretty much align with the views I've had from reading reviews, but it's massively more reassuring to get a trusted opinion.