Q: check engine light for o2 sensor because the straight pipe and an Evap code are on, what does this mean, want to buy but unsure

It’s possible that the seller eliminated the catalytic converter and along with it the downstream oxygen sensor, which is illegal by the way, and also guarantees a fail in any jurisdiction where emissions testing is mandatory. Unfortunately, an as bad consequence is now that the check engine light is illuminated continuously, it is quite literally impossible to get advice of "new" malfunctions that might arise, simply because the light is already on and thus new malfunctions become invisible. That’s not a good circumstance from the standpoint of safe, economical and reliable operation of the vehicle. The EVAP system issue could be as simple as a faulty gas cap or as complex, and costly, as a rusted out gas tank or rusted fuel vapor lines with pinhole leaks. As is, be sure to take $1,000 or more right off the top because you are going to need a Subaru OEM catalytic converter, an oxygen sensor, and possibly other exhaust components in order to restore the car to legal condition. Add to that the cost to repair the EVAP system malfunction. The car might be worth buying but you have to discount it to be sure the inevitable costs are covered. At present, there is an oversupply of vehicles (too many cars, too few buyers), so there is plenty of room to negotiate.

Inasmuch as used car repair, maintenance, and restoration costs are highly dependent on the actual, as found, condition of the vehicle, a way to get a reasonably precise estimate of costs you might face, and also to protect yourself in the transaction, is to request a pre-purchase vehicle inspection. That inspection, carried out by a certified Mechanic, dispatched to your location, will give you lots of data that you can rely on to make a fully informed, intelligent decision and minimize your risk in buying a used car. If you look at the just cited link for YourMechanic’s inspection service, you will see a long list of the vehicle systems that must be inspected as well as details of YourMechanic’s inspection methodology. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.