Depends how much you like the car and If you can afford the payments on a new one.
Knock sensor is easy to replace, even for someone not super mechanically inclined, all you need is a socket wrench.
o2 sensors are harder but not ridiculously so
Look for parts prices here to give you an idea what they cost.
o2 sensors can be had cheaper at napa or similar
They should not charge you more than 1/2 hour labor for the knock sensor [more like 15 minutes] and a 1/2-1 hour labor for both o2 sensors [if it takes the mechanic more than a 1/2 hour to rack the car and change both sensors, he's not a very good mechanic ... I can do it on my back in the driveway in that time]
How much oil is it leaking ? if it's a small leak that's just sort of a messy nuisance, clean it off every couple months and don't worry about it. If it's a lot and you're losing quite a bit between oil changes, you should reseal it. I would have to go look at the book but, I thought you just had to loosen the motor mounts and lift the motor a little to have enough room to pull the pan ... don't quote me on that yet.
not sure why you're losing coolant, any signs of head gasket failure ?
Coolant does not disappear, it either leaks out somewhere or gets blown through the motor out the exhaust or mixes with the oil through a blown head gasket.
check for bubbles in the res, oil in the coolant, coolant in the oil, sweet smell and green drips at the exhaust pipe. Any of those are signs of a head gasket failure.
If the head gasket is blown it's a pricey repair and may put you over the edge of wanting to keep the car. If you can trade it in, that's up to you ... the dealer should know enough to check for a head gasket failure and if he doesn't, his problem. Personally I don't sell anything private party without disclosing known problems and I think anyone who does is an A$#$*%&#.
No idea what the chirping is, are the boots torn ? grease everywhere around the inner or outer boot ?