Here's my take, since i've done almost all this before
OEM battery is 100% worth it
Water Pump/Thermostat should be a little more than half that price
Coolant flush is a $10 too much
Oil filter housing gasket is a $10 part and maybe 4 hours labor, Maximum price $550
Underbody panels? the pastic ones? they've been missing on my car for years.
2000 528i Sport Premium 5speed