As somebody who made both of the "absolutely terrible suggestions", I will elaborate on my logicI mean, the crux of this thread is an outlay of £105 (on something that you could easily resell later and get most of that back). If that's a life-changing amount of money for you, I'd suggest that perhaps it's not the time to be buying a gaming PC at all. It really shouldn't be a huge decision.
There are also some absolutely terrible suggestions in this thread, like not spending £105 for a GTX 1080 that'd give you a perfectly decent gaming experience and still play just about anything at a reasonable framerate with some upscaling, but rather just use the integrated graphics which have all the power of a mouldy potato and will struggle to run 15 year old titles acceptably. Or downgrade everything in your build, going from a 9800X3D to a 7600 as if they're even remotely comparable, all so that you can have the awesome power of a 9060 XT (a card about twice as fast as a GTX 1080 for more than triple the outlay). You're far better off getting the base of your system sorted first time around. It's FAR more hassle and faff to go about upgrading your CPU/motherboard later compared to your graphics card. Buying a CPU that's already low-end for the platform along with the crappiest old B650 motherboard you can find is a terrible idea if you actually plan to keep this build for a good length of time (and there's really no reason that shouldn't be your plan). There's nothing wrong with budget builds IF that's what you want, but I doubt you'd be considering a 9800X3D in the first place if you wanted a budget build. Skimping on the bones of your system will only lead to unhappiness later if it's not what you want.
As for worrying about things not working, everything comes with a warranty and right to return. Just don't go buying parts months apart to make that more difficult. Buy the thing, build it, test it (you don't even need the graphics card for that part) and then you have returns windows and peace of mind.

I don't think the crux of the thread is the £100, I think the crux of the thread is ALL the monies.
RE: the part about the low-end CPU with the crappiest B650 motherboard. I do take your point, but we see it a lot in the build section where somebody gets the latest high-end parts (like 9800X3D and 14900K) just 'cos that's what they think you need for gaming. The parts I suggested are perfectly fine for a decent 1440p build for the next 2-3 years, which if the OP is uncomfortable with their overall spend (that's my impression), then why not?
That board really isn't that bad either, it'll take an upgrade to a 9800X3D (or whatever else in the future).
The integrated graphics suggestion was in response to the OP saying they just want to make sure everything works before buying anything else expensive.