I just bought my friend's 1999 Civic LX with the auto D16Y7 and 241k miles for $300. Runs, drives, no obvious major problems. It has a pretty detailed service record and has been mostly well maintained. This car is supposed to be my reliable daily driver, nothing else, no builds.
So I've never owned an operational vehicle before, and as such I'm hyper paranoid. I have a ton of head knowledge about every car subject imaginable, but virtually no hands on experience.
Every tiny thing freaks me out. It leaks oil from somewhere in the top end and I assume it's the head gasket. I see 2 tiny bubbles on the dipstick I can't decide if it's totally normal bubbles, bearing material, or froth from having too much oil or whatever else.
The transmission shifts a little hard every now and again, signalling a need for a fluid change. I assume it's catastrophic failure on the way.
It cranked and started at way too low an rpm so I had to gas it to get it fired up, and after it idled and ran just fine.
The point is I get too freaked out. It's high mileage and I'm terrified something catastrophic will happen.
I'm asking for some kind of advice on what to do to make it even more reliable, reassurance, as well as knowledge about common issues I might face.
I'd appreciate it.

 

Change the distributor o ring.

 

It's a $300 car. Just do some preventative maintenance and don't worry about it.

How exactly do you have a ton of knowledge on vehicles but no actual hands on experience? That seems...counterintuitive.