If you can afford the extra $200-300/month go for the M5. You won't have any need to mod really. I've spent in past 5 months of owning new car about $4k on Dinan, then tune, spacers and resonator delete and some ancillary items.
I just always deny to myself that I'm going to end up modding my cars, 4 weeks later I'm researching them, and I have to keep my mods secret from fiancé as I'm always telling her we're on a budget
If your tastes are like mine perhaps look at 2019 M5's or take over a lease that has 12-24 months left. I keep pondering the idea of putting mine up on one of those sites and taking over a lease on the f90 M5.
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2013 535i M, Carbon black 20' 6 M wheels, totaled.
2018 750M ,
2013 M5 , Individual Silver, Bolt ons-Straight piped
2019 M550 686BK Black Chrome 20"wheels, filters, Evolve Stage 1, Res delete
2024 i5 M60 Cape York Green