Buy a new computer (or car, or clothing, or garden shovel) when you need it. If you think you can afford to wait for two years, you don't need a new computer now. If your current computer isn't meeting your needs, buy one now. Don't worry about the unknowable future, deal with the known present - the use/enjoyment you'll get from that new computer over the next two years.
As to all those, "Get an SSD" comments... Maybe the OP needs 1 TB of internal storage and doesn't have the budget for 1 TB of Flash - he is, after all, asking about the entry-level 27" configuration. Maybe the OP needs the performance of SSD, maybe not. Telling someone to buy something because that's what you'd prefer for yourself, with no regard to that person's needs or budget... not very helpful.
That £1750 model comes with a 1 TB Fusion Drive. Upgrading to 1 TB SSD would add £450 to the price. Maybe that's justifiable/affordable, maybe not. 256 GB SSD would add just £90, and if that's all the storage the OP requires, great. If the OP really needs 1 TB... then he has to spend time and effort managing an external HD along with the internal. If that external is a spinning HDD then it'll have the same long-term reliability risk as an internal spinner.
I've been using a Mac with Fusion Drive since early 2014. I have had a great experience with it. Nearly everyone who says, "Don't get one" has no experience with it. They theorize about what might go wrong with it, but they do not have experience to back up those theories. Or they say, "It's not as good as SSD" as if SSD was the only other option. No, there are three options - slow spinning HDD, fast SSD, and Fusion (hybrid drive). If one has the money, by all means, go all-SSD. It's certainly the best option of the three. But Fusion delivers, to the vast majority of users (there are always exceptions), far better performance than spinning HDD for just a slightly higher price than spinning HDD. It is a great, reliable (yes, reliable) method for improving the performance of a lower-priced computer.
