Sounds like they want to do a full "top end rebuild" including redoing valve seats, replacing valve guides and seals. You could cut the bill in half or more at an Indy and just do the seals if rest is within spec. They can do a leak down test to determine how well valves fit against the seats and during work measure wear of guides to determine if those need replacing.

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2008 535i - Monaco Blue/Tan Leather, Sport, Premium, Cold (+ heated rear seats), Navi, HUD, PDC, CA, Sunshades, Fold down rear seats, CPO, Extended Maintenance