It's rumoured that Apple are going to abandon Intel, and switch to something else (quite likely their own CPU design, quite possibly heavily based on ARM). They done this before of course, 68000->PowerPC->Intel. I've no idea when this is going to happen. However, it's something to bear in mind; a Mac anything might be very much "old" in just a couple of year's time. Last time Apple did do a reasonably good job of supporting old PowerPC software on new Intel Macs, and to some extent vice versa. There's also a chance I suppose that the Mac family might be split across Intel and their own thing for a while, further blurring the cross-over line.
What does this mean for a prospective purchaser? It's hard to say. I've no idea when this will actually happen, if it happens at all. Arguably it's worth cracking on and getting a machine ASAP, so that when the big switch over happens and the support clock starts ticking down on your machine, you'll have had that little bit of earlier / extra use out of it. If I were in the market myself, I might perhaps not buy one of the very top end machines (unless I had a very good reason to), so that if the switch happens sooner rather than later I wouldn't feel like I'd blown quite so much cash on something that I'd then be stuck with.
I'd like to reinforce other's comments about keyboards. It's well worth avoiding anything with the butterfly keyboard. If you're buying brand new today, their entire current line-up has now reverted to the much more resilient previous design, much to everyone's relief. If you're buying second hand, you basically have to go back to 2015-ish to avoid the butterfly keyboard. That can be surprisingly successful - I bought a 2015 Macbook Pro 13" off ebay from an outfit that had a good rep in refurbed models, and it's been fantastic value for money. Ironically, they were willing to throw in a longer warranty that Apple give you from new...