@garyi , I'm a bit late to the party here but have extensive knowledge of your issues. Firstly ignore the runflat comments, if you're happy with them then move on as they're really not the big issue some people make them out to be. Secondly Merc and BMW have a history with easy to damage OEM alloys that other manufacturers such as Porsche just don't have. Make of that what you will. In your situation I would be looking to either:
source two replacements second hand. What I do when I buy like this is always buy from a company and have them take proper images of both sides of the wheel and ask them to run the wheel on a balancer and check for any buckling. Get them to email this info and tell them if they prove to be unusable you will want a full refund. the reputable ones will have no issue doing this.
OR
find 4 aftermarket wheels second-hand preferably, but be careful. Stick to decent well know brands such as BBS, Ronal and the likes as cheaper wheels could be as bad if not worse than your OEMs. The recognised brands will be forged alloys and much stronger. If you don't recognise a brand do a bit of Googling and the car forums will reveal all. BBS and the likes are expensive new (although not as much as the OEM) hence why I suggest second-hand. What you need to check here is that the wheel size and width but also the offset... this is crucial. The offset is the distance from a wheel's hub mounting surface to the centreline of the wheel. See this diagram:
You will need the offset of your current wheels which are stamped on the back or can be gleaned from the specific model on this database:
BMW OEM Wheels
If you get a set of wheels where the offset is massively different you will get handling issues and possible rubbing of the wheels on the arches.
As an illustration of what I am saying I had a lovely SL55 AMG and put the expensive split rim OEM AMG alloys on it (£3K for a set - secondhand). Bent two irreparably in 12 months. Replaced with BBS forged CH-Rs - were still on when I sold the car 7 years later and in perfect condition.
Feel free to PM me if you want any further specific help.
