Dear Anxious,
I read with great intrest in your experience, and I must say you are one patient person. One in dealing with your troubles and two in documenting it all down. I for one never look at the milage of a car when I do purchase one, the milage is only a tell tail of when to change what. If a proper maintenance is done then you wouldn't have much problems, but I'm sure you already know that.
I have a e36 328 which is 11 years and have clocked 317000Km on it. However I don't think you spent overly on your car, you just spent a lot in a very short period. I cracked head was defenately a bummer, but that's life. I have a few suggestion if you'd like but be warned, I'm no expert. these were issues I had with my ride and what I did to fix it.
1. you describes the thug..thug sound from the dash every few seconds. did it sound like something closing? It clould be your AC air intake flap, the pin may have come loose.
2. When you slow down your car the engine sounds loud. Do you feel a loss of power? Had that, changed my batteries and did not have that problem anymore. Could be something else I don't know but it sure solved my problem.
I'm a little kiasu as well when it comes to cars, I change oil every month regardless of milage, but never exceed the recomended oil change milage. i budget about RM1K a month, and when my service do not reach that amount i look for something to change, or add. I've also learnt something from mechanics. If they are close to you, everything is fine with your car. You have to sometimes force them to change things. there was one time I felt my car floating, asked my regular mech about it and he said he can't feel it. Told him to check lower arm bushings he said not time to change yet. After much persistance, when he removed the lower arm mounts, they were indeed worn.
I guess at times the mech is not the best place for advise. that is why I'm in the forum, there is a lot of invaluable information here, and nothing beats the experience or a fellow driver.
Well, i still have a problem with my long shaft which I can't seem to fix. Other than that the car purrs like a pussycat.
I hope this will be the last of your problems, and it will be nothing but sheer pleasure after this. But one word of advise, Change parts when required and before they wear off, it'll save you loads of cash.
cheers Mate