Ok so. Is it just certain apps running slowly? Unsurprising you mention the Microsoft apps; I've had issues with such for nearly twenty years of running Office on Macs. I use the Apple versions, Pages etc, now. But I'm not a heavy user of such software tbh. It's worth running
Activity Monitor, to see if any apps are hogging resources. It is likely, that as apps get updated, that there might be conflicts with vestiges of older versions on your machine. You can see how apps use the CPU/s, although I think it's mainly the more graphic intensive apps that use that more. you've certainly got ample RAM for running Office. Or it could just be that your HDD is wearing out a bit, or getting to full, something like that.
Upgrading to a new drive isn't that painful, and needn't be expensive, as you can DIY. As mentioned above; iMac screens are held in place with magnets. There are various YT videos which explain how to remove it safely. You'll need a couple of rubber suckers, is all. You may need a drive caddy depending on the type and size of internal HDD already in the iMac. But it's little more expense than the SSD itself really. Look on BT shop or Novatech etc, for good deals on SSDs. A 1TB drive can be had for as little as
£80 or so, these days.
Eighty pounds. Not four hundred.
You will of course need to backup your existing drive and system, even if you want to do a fully clean install (recommended). You can use an external drive to backup using
TimeMachine, then a reinstall from that, transfer files over etc, once you've put the new drive in. Again, info on internet regarding this. A clean install is the best way, cos it'll remove the rubbish that may have been slowing things down. A SSD Vs a HDD or even fusion drive, will be
sig-NIF-icantly faster, and speed things up no end. My 14 year old MacPro fully boots up from power off, in just 20 seconds, and that's with a RAID HDD pair and another drive with an older OS on it. The iMac I restored with a new SSD was similar in boot up time, and just so much faster than before.
To back up all your existing files, I'd really recommend at least a twin-drive NAS/USB drive, with two
RAID O 'mirrored' HDDs for redundancy. So if one fails, there's always a second one for backup. Or if you're just using your existing internal drive for al storage, at least get an external USB drive to backup all files onto that as well. Always have a backup. And it's a good idea to back up the back up; I once had 2 HDDs fail within days of each other; fortunately I had a RAID 0 set, or I would basically have lost my 'life's work' of digital files.
But TL;DR;
you don't need a new computer. Your existing one will be fine, with a bit of tlc. To replace your current iMac with an equivalently current spec PC, you'll be looking at £1500+ anyway, so that's into iMac territory. Why switch when you don't have to?
It's a good idea to read up on such things regarding replacing drives, SSDs, backups, RAID etc. you can always refer back to this thread for more advice, I'm sure there are others with far greater knowledge and expertise than me.