If your partner doesn't want a manual they likely wont want an SMG M5.
Mechanical insurance is exceptionally expensive for these. You could be looking at $2200 per year. Mine cost that much and was limited at 2 years.
Before you own an M5, ask yourself these questions:
1: Can you live with 20L/100km fuel consumption?
2: Will anyone that frequently travels in the car have issues with the SMG? My wife won't drive mine.
3: Do you have cash flow available IF something was to go wrong? I.e. can you pull $5-10k of cash together if the worst was to happen?
4: Are you happy with ~$2-4k per year servicing costs?
If you can get past the points above and you want an M5 then go for gold. Otherwise I would suggest looking at alternatives such as the newer Audi S6 V8 twin turbo, AMG E63. They aren't an E60 M5, but they are far more modern, faster and in the case of the Audi, fuel efficient.
Common maintenance items:
Brakes: DIY @ $2500
Oil/filter: DIY @ $250
Spark plugs: DIY @ $200
VANOS Solenoids: $1400 each * 4