I hope you got a really good deal. A new key is dealer only, as are remotes. Expect to be charged north of $300 for both.

Depending on the mileage, look for a timing belt replacement sticker in the engine compartment.

Both seats, or just the driver's? If the seat switches got wet, they could be corroded internally and are difficult to take apart and clean - lots of very small parts. If there is a pick&pull junk yard close to you, you might want to try a replacement from another S60. Also check the plug under the front of the seat.

Dealers are not cheap. Some do very good work but that depends on the day of the week and the mechanic involved. If you go to a dealer to get a new key and/or remote, you might want to pay for a complete inspection. That way you have a list of any potential problems and you can decide if and when you want to tackle them.

Remember, do not let a shop use a machine to flush the ATF. The high pressure will blow internal seals.
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My back feels better when I sit in a Volvo seat