I've just gone through a bit of an upgrade by raiding the pile of Weee we have at work.
I've gotten myself an i7 9700, 16gb DDR4 and a 512gb M2 SSD.  I've added my R9 290 graphics card which I had to flash for UEFI compatibility, and moved across my 4 and 10gb mechanical drives.
I'm looking at upgrading my graphics card now though, as I suspect it's a bit long in the tooth.
What's going to give me a good upgrade without spending out on the frankly ridiculous prices of todays graphics cards?

Depends what sort of games and settings you're intending on, but I'm about to sell my old 1660 GTX which is probably about 50% faster than your card but still runs all the latest games fine on non-insane settings. On ebay they seem to be going from £100-£150 depending on whether its standard, Ti, Super, the condition, etc. I've offered Bacon first dibs but if he doesn't want it I'm happy to offer it you, it's not been used for mining or anything stenuous at all.

I caved.  Bought a 2080ti off ebay.
I figured I've got enough bits kicking about to build another PC and then sell it on and recoup.

I'm only really looking at 1440p gaming, I have a 34" ultrawide.  I just don't like being told "LOW" when loading a game

2080ti is still a very capable card, nice one

Sorry for all the questions but I trust no one else!

I need to buy a 2Tb M.2 drive for this rig (mainly to house the growing monster that is MSFS!) - what type works with this board?

M.2 NVMe PCIe 4.0 or just M.2 NVMe?  Or neither (or both)  Yikes! 

I'm really struggling with the terminologies here.

Will this fit and work?
https://www.scan.co.uk/products/2tb-samsung-970-evo-plus-m2-2280-pcie-30-x4-nvme-ssd-mlc-v-nand-3500mb-s-read-3300mb-s-write-620k-56

M.2 is the form factor, basically the type of connector. NVMe is like a protocol, more akin to SATA, your board - from what I could see from the specs after checking - it does in fact supports NVMe PCIe Gen3 x4 M.2
You want something like this: https://www.amazon.co.uk/dp/B09JM8DJNS?smid=A3P5ROKL5A1OLE&tag=pepperugc03-21&ascsubtag=2515421466

Other vendors, products, etc are available of course, that was the first Google result

Many thanks - so is the one I linked to okay?

I know it's way more expensive but I have trust issues with WD not to mention my OCD (all my other drives are Samsung EVOs! 

...and do they need heatsinks?

The Samsung is the business and it already has heatspreaders

Haha, I bottled it and bought a Samsung 2Tb SSD!   
Known quanity and I wasn't at all confident the M.2 would fit given the other addons and CPU cooler mount I have in there.
Ah well.  Let's see what happens in December!

Fiiiiiinal question here:

My mobo manual says it supports DDR4 RAM up to 2133Mhz.  Is that it?  DDR4 is available at much higher speeds is it not?  Maybe that's all that was available at the time?...

Any clue as to what I could go for now?  Just looking to boost this rig a bit before the build in Dec.

honestly in my experience ram speeds make naff all difference... gave up buying the fastest stuff years ago

Okay, fair enough. 
For a total of 32GB, would I be better off getting two matched pairs (4x8Gb) of just one (2x16Gb)?

4x8 is better :-)

I thought that there's a bigger likelihood of instability with four?  And stresses the CPU's controller a little more?