So I got a new job and will be commuting roughly 600 miles a week. What can I do to ensure the functional response of my car for the many miles ahead? Currently it has about 177k miles on the stock engine.

Here is a list of some of the things I think will need to be changed:
- Head gasket
- Waterpump?
- fuel filter?
- spark plugs
- Brakes

Here is a list of some of the things that I have already changed:
- Thermostat
- radiator
- Tires
- coolant temperature sensor
- Oxygen sensor
- catalytic converter
- Timing belt was changed at 148K

Any suggestions of what else needs to be changed?

well first off what car do you have?

you dont need to be changing a head gasket unless there is a problem.

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98 st204 in california.

I'm going to be driving from sac to san jose and to san francisco, and I want to protect her as much as I can.

Well if you do the head gasket, you might as well pull the engine and get the head and block decked so that way they're machined flat so you won't have issues with the new head gasket.
Waterpump should have been replaced when the time belt was done.

You're really over thinking things. Just get a new distributor coil, cap, rotor, plugs, and wires, keep up on your oil changes every 4k or 5k (which ever you prefer).
If your car is having fuel issues then replace the filter? I believe I still have the OEM one in my car at 279k.
You only need to change your pads and rotors if A) your pads are low (just replace pads) or B) your rotors are bad. If the rotors are bad, do rotors and pads all around.

Maybe change your power steering and alternator belts too if they're getting worn out?

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See that's the exact response I want to hear, someone to slap me and tell me I'm being paranoid. Thank you. Is it bad that I change my oil at 3k? The last time I changed at 5k I noticed burnt oil on my oil cap, and the last time I did 4k the oil was just a hair blacker than I like.

The dealership I went to was kind of shady, I noticed the belt was bad before I bought it so they said they would fix it, and I sincerely doubt they changed the water pump. I haven't changed the power steering or alternator belt. The brake feel is just not correct, hard to describe, on normal day to day breaking its adequate at best, but on hard stops it doesn't inspire confidence at all. The rotors might have some life to them but I don't think it's been changed ever, so I don't know how much meat is left in them. The pads don't squeak, but they just don't bite.

My fuel economy is excellent, my practice commute to work and all around the bay area lead to a whopping 32 mpg. Mostly highway miles