With the prices of M5's the way they are, there is 0 sense to not wait a little and jump on an M5. In fact, certain things are a bit more reliable/inexpensive to repair on the M5 Vs the 540 (alternator, less vanos issues, less timing chain issues).
There are many places that rebuild the Vanos for a few hundred dollars.
My 6/00 updated ring/vanos M5 is at ~230K on original Vanos. My wife's 6/99 ~220k M5's vanos was re-freshed about 3-4 years ago.
Both experience the immeadiate chatter when you first start it in the morning but it goes away within a few seconds (once the oil circulates and lubricates evenly).
This spring, they are both getting upper chain tensioners are preventative maintenance, and that should actually help with the immediate chatter first thing in the morning. If there are non SES Vanos codes and the car isn't shaking profusely and rattling non stop and not down on power, I wouldn't worry about the Vanos.
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--'00 BMW ///M5 .:Jet BLK/BLK:. Individual - Style 65 Wheels; 9.5" Square Set up;275 Tires;10mm/3mm Spacers; Black Kidneys;Muffler Delete; 3.5" Dual Tips; Beast Power; "Dinan" Ducts; E60 SSK; BC Coils
--'00 BMW ///M5 .:Avus Blue/Grey/Black:. '01 Exterior Upgrades; E60 SSK
'81 MB 500 SEC Euro Koenig
'91 Alfa Romer 164S- Red/Black/ Recaro interior