@TBP88 , I totally hear you ... R25k repair bill does annoy my day as I've been spending roughly around that every year on one of my cars ... it is frustrating especially on months where you gotta spend on a birthday or anniversary... I always looked at the last of the naturally aspirated engines as everything has a turbo these days .... dont get me wrong , they are more efficient and much faster however there isnt much joy driving them ... I have a Renault clio R27 , and the amount of smiles per km outweighs the newer clio RS18 that I drove ... but also something that will cost around 200k in maintenance is ridiculous on a long run ... I believe the vanos system is a nightmare.
Everything is well documented so there are no real nightmares or unknowns anymore, as long as you follow the maintenance schedule and have money... and a lot of it. The latter is what is not very appetising for most people.
I often joke that I bought my F10 M5 because I was too poor to run a V10

. If you can, try to go and drive one to get it out of your system as well. After being quite enthusiastic about 458s and GT3s and F430s the past few weeks I am kind of off them having my F90 back and doing a few runs in my Alfa for instance. Less that I don't want them and more that I would have to give up two perfectly good cars, both of which I've owned for years now and know the full histories of.
I would suggest you go and drive some special turbo cars as well. If you are saying you don't get much joy driving them I can only assume you've driven the wrong ones. You can go and drive these NA cars and there is a mystique to them and yes some of them are brilliant... but there is an insanity to a turbo car that brings its own kind of joy. To me, they feel visceral in some sense, very refined in others and with a great soundtrack but also, frankly, they feel like donkeys alot of the time (especially at altitude).
The miles per smile in your R27 that you mention (one of my favourite little cars BTW) is also going to be coupled with the relatively carefree nature of owning and driving something like that vs. the existential dread that comes with driving something that is very expensive to keep going. You are getting proper exotic bills with the S85.
Whether you have lots of money or not, spending the value of a holiday for 4 in Mauritius is not likely to make your Mrs very happy. Only at the truly FU levels of money that
@TBP88 mentions will you truly be carefree with these cars IMHO and even then it may be irritating. You also need a specialist to work on it since BMW might not have techs who have ever seen one let alone worked on one depending on where you are based.
Avoid getting caught up in the latest "last of" hype as well. This "last of" story has been happening since 2005 across many brands as has "6MT or you're a lazy casual". The particular one for M5s has been "Last manual" for our market with the E39, "Last NA" for the E60, "Last RWD" for F10 and last non-electrified with the F90. At every single point people have spoken about "analogueness" or other 'x' factors because even this is a moving target until the next thing comes out. With M5s, you can make a case for owning any of them though - there is something special in each of them. Even the G90 - the only reason not to buy one for the use case most M5 owners have is if you already have an F90.
I love the E39 M5. It's actually my favourite BMW. You could get them for under 200K before someone anointed them as God's chosen M5... but even in the 400s a few years back they were in conditions that made no sense. The other bit is this stupidity of "the new cars are not as good" that follows the increasing of prices for 20 year+ old cars beyond the prices of their 4 generation newer modern counterparts. The only people saying that are a) people who have them and b) people who are trying to sell them. At today's prices if you had to tell me I could do a straight swap of my F90 for any of the E39s asking stupid money, I would show you the door... but that is the reality -there are E39s asking more than I would realistically get for my F90...
My feeling is that your budget for an E60 over a 5 year period is going to be about 600K including buying the car itself. You are trading a 20K monthly installment for something newer, faster, more reliable etc for a 50-100K bill now and then... on a car that will likely never be worth more than 450-500K on its best day, UNLESS bought at very low miles and then pickled.