Ok, on the brakes, the abs wheel sensors are in the hub bearing assemblies, so replacing the pads and rotors on the front and rear will not affect or damage the sensors. About the only way you could damage them is to drop the car onto its hub bearings while the wheels and rotors are off. Other than that, you cant damage them doing the brake job as you wont come in contact with them.
As far as brand of brakes, I would recommend either Akebono or Brembo pads and rotors. They both are top of the line and will increase all aspects of braking performance- from stopping distance, quietness, and longevity of brakes as well as reduced brake dust.
Before replace the converters, I would suggest having a shop with an exhaust analyzer test the exhaust for proper emissions. First, bad converters set the check engine light and very specific stored fault codes(P0420, P0430, P0421, and P0431). Unless your mechanic mentioned these codes to you, the chances are very slim that they are needed.
Poor mileage and sluggish acceleration can be dirty fuel filter, weak fuel pump, weak/fouled spark plugs, vacuum leak, or dirty fuel injectors, or even associated with the overheating. This problem needs further diagnostic work before spending money needlessly on parts that are more than likely good.
The overheating can result in leaking headgaskets, cracked cylinder heads, failed water pump and failed thermostat. The cooling system needs to be pressure tested for holding pressure. Lost of pressure with no external leak confirms leaking headgasket or cracked head. The fluctuating temp gauge is probably due to damage to the engine coolant temperature sensor when it initially ran hot. Again, further diagnostic work is needed before sinking money into parts that could be good.