Guides and seals are an M103 plague (yours is the M104), but a good shop will pretty much recondition the heads for around $100.

There is a lot of room in the engine compartment, so it's a good time to check other items that may need work...especially cleaning out the EGR tube, which is difficult to access when the engine is buttoned up.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car