Hi all, bought a beater Corolla today

I bought a beater 2000 Corolla today for 400 bucks. It's a 4 door 5 speed with 442000 kilometers but runs nice. Of course, a 16 year old car with a quarter million miles on it is going to have some problems, and mine does have a few hopefully non serious ones.

Firstly, the clutch pedal feels mushy. The clutch itself seems decent enough. I've had 2 other high mileage Corollas (88 wagon and a 93 wagon) both with 5 speeds and I don't recall the clutch pedals on those feeling mushy.

Secondly, I have a few trouble codes. P0125, P0131 and P0141. These all look to be O2 sensor related according to Google. I cleared the codes then went for a short drive and the P0125 popped up again but the other 2 haven't yet. So, where to start on that? Fuel mileage looks like it's right around the EPA estimate but I wonder if there's any improvement to be had if those three codes were fixed

Those are the main issues. There are some other small things, AC inoperative, front end needs alignment, funny sounding horn but overall the car seems to be fair enough.
Id start with the slave cylinder, there inexpesive and often over looked when the clutch pedal feels mushy. If the clutch doesnt disengage untill the pedal is almost on the floor it could be a worn clutch
Def slave cyl if it hasn't been done recently. Have a question though, have you owned Honda or Toyota in the past?? Lots of people that haven't often complain of poor pedal feel. I'm not saying there isn't an issue with yours, but I've driven a number of Honda and Toyotas even after getting a brand new clutch, slave cyl, throwout bearing and even clutch master cyl and the pedals still just feel flat and sloppy compared to other brands. I'd say as long as the clutch is functioning top notch like it's supposed to, no need to create work.