What must be done to keep this car running?

It depends on what kind of mechanic you are , I wouldn't put any $$ into that even if I had tools , shade tree and a case of beer , , I 'd trade it in or get what I could for it and get something else , a car is maintenance and $$ and that;s all there is to it

Unless it has to pass emissions testing neither of those things need to be done. Fresh spark plugs would likely make a noticeable difference . Change the serpentine belt if it’s breaking down. Give it an oil change and set the rest of the money aside for whatever might come up. The car is nearing the end of its life so I wouldn’t spend money on non essentials at this point. Just drive it till it’s dead. That’s the cheapest transportation there is for you right now.

I have to try to keep this car running for 3 more years or another 150k. I understand that may (haha) not happen... but I'd like to try. Oil changes I've kept up with, brakes done as needed, serp belt changed last month along with alternator. I'm not any sort of mechanic, but someone who was never taught anything about cars except how to put on a spare tire. Does it need a transmission drain and fill? What a about a coolant flush? How important are those things compared to replacing shocks and tie rods and the like? I'll get the spark plugs done, thank you for the suggestion.

suspension is important , if the parts are worn it can ruin tires and may be unsafe , coolant flush if it's real crappy looking with sediment , tranny flush if fluid is burnt smelling , a religious icon on the dash board maybe that could help it last

Those are good suggestions. Actually changing the coolant should be done even if what is in there looks fine. It does have a life span and is probably due. If it’s an automatic the same thing applies, change the fluid and filter. After that save up your money as best you can and wait for things to break or require work, which they will on an older car. Sounds like you’ve taken reasonably good care of it up till now. They are good cars and with some care you might make your goal. Mine has 150,000 mi. on it and I plan on another 60,000 before retiring it.

I have a 175000 miles on mine; with all original parts. Also note there were recalls on the 2005 cobalt

225,000 miles on my 2005 Cobalt. You should change your coil pack,. The two control arm bushings. The O2 sensors, one before and one after the catalytic converter should be changed if you're getting a code/yellow engine light for them. Changing mine improved the gas mileage. Expect your gas line near the driver side rear tire to rust through and leak if it snows where you live. If you have an automatic transmission change the fluid. Change the coolant. I've had to change the sway bar links 3 times. The struts one time and the coil pack twice. My yellow check engine light is on to replace the catalytic converter and has been for 60,000 miles. I don't have emission testing and it hasn't effected gas mileage.

Guru how much do you think you put into your cobalt

It's a pretty old thread Zae. Won't likely get much response here. I take it you have a Cobalt?

I put head gasket sealer in my car and now it will not start why?

Scan it for trouble codes. I doubt that it has anything to do with the sealer.

I have a 2006 and its sitting at 278km. Still going strong, regular maintenance work, But now needs a new wheel barring. Also slow to climb speed...would this be break drag or a fuel issue?

It's not brake drag and is not related to needing a wheel bearing. It's most likely an electronic or engine related maintenance issue. Scan it for trouble codes. Check the spark plugs. It's easy to overlook them these days.