Mk3.5 TX wheel change

Hey all. I bought my Mk3.5 used and it came with optional 18” alloys (Ford part 1893233) which are 8J x 18”, offset 55, tyre 235/40R18. The “standard” wheel (Ford part 2238239) which are 7Jx17”, offset 50, tyre 215/50R17 (oddly the Ford Pass app shows my car as having the standard wheels).
 

I’m thinking of either eBaying or shelling out for some genuine wheels to get a bit more tyre (and also cover up how pathetically small the front discs and rear drums are!) for a little more comfort. I also used an online calculator that says the difference in rolling circumference will be 5mm resulting in a measly -0.25% Speedo change so would read 59.85mph at what was 60mph.

First - I’m presuming these would actually be a straight swap, wouldn’t need to be getting spacers or anything? Second, would I actually notice much difference? Because there’s sets of 4 genuine new for £600 on eBay, then add on £400 for new tyres and probably £200 for TPMS valves and that’s a fair wedge, even if I can recoup some cost selling on these 18s and tyres.

Thanks all!