Will 215/40R18 fit on stock r53

you probably can without doing anything else, the factory 18's for example wear 205/40r18 anyway, the rolling circumference being similar to an otherwise stock 205/45r17, then consider that the 17's on stock et48 rims will take both 215/45r17 and even 225/45r17 without fouling anywhere on a standard car so no work is essential there, they will sit close to the inner arch liner at the back but still miss.

205/40 = 82mm wall vs 215/40 = 86mm wall, not going to be an issue in that regard and will actually slightly improve the factory speedo over read.

As far as 205 vs 215, you also should consider all tyres are different overall widths and carcass profiles anyway, so you might be quite surprised just how wide (or narrow) different makes can be. Semi slicks are a great example, significantly wider carcasses at the tread, if it helps, the above sizes quoted are known to fit in such tyre versions, so I really doubt you'll have any issue at all, depends if you wheel offset affects things more, or vehicle height if it's been altered ;O)

yes most OEM are 7j and also et48 in the main so you're right to consider this, the way I see it, your offset difference being just below 50% of the additional 'J' (width in inches) will mean the rim is slightly further inward compared to stock, offset being the distance between the midway point of the rim bead faces vs the mounting face of of the rim, lower = outward, higher = inward, so if an inch in millimetres is 25.4 and your offset being 40 has moved the 'J' midway point 8mm further out, the rim is still 12.7mm wider resulting in it being actually 4.7mm further in at the rim yes? I guess hence why a small spacer has been added, 5mm perhaps but possibly not actually necessary? Although fyi these are better in a hubcentric style which helps locate the wheel centre bore against the hub flange, iirc these might only be available in 6mm, personally I'd use those if any but you may not have scope. Ultimately if you have these size spacers all the above would apply in regard to the clearance to the inner arches and front struts etc, but not necessarily to the outside which of course is further out.

Unfortunately whilst it might read that I'm familiar with a variety of rim sizes on these I'm actually not, I prefer to use OEM wheels myself although I do currently have 16mm spacers under the fronts and 11mm under the rears. I do have aftermarket lighter options in a lower offset (37), just 7J still so they don't pose a problem to me particularly, well they do a little as I'd prefer et42 to cater for my 225/45r17 tyre options, not a fan of 'poke'.

Anyway, I hope the above makes some sense, no doubt someone will be along that has indeed run all sorts. Maybe be mindful of when people have run them and they say they're ok, try to get pictures of their fitment and also try to see what they did to their geometry, as some owners cut trims away, experience other fouling, have tyre or rim poke which they think looks great, and quite often can set fairly extreme negative camber in an effort just to get the wheels they want to fit. Not something I'm a fan of at all and would never recommend fwiw.

Still, your planned tyre size is only +10mm width, and on a wider rim already in play plus the above regarding different brand tyre widths and when only talking about a relatively roomy 205 when on a stock rim anyway, to then a 215 being mounted on this inch wider rim, the real difference at the tyre might not be that much at all perhaps? Hope it works out ;O)