If you question whether you are qualified, I would not be poking around in the electrical cabinet for more than a loose wire.
In regards to the E-stop errors, a schematic will show how these are tied into the circuit loop. With a bit of logic, you can jumper across each Normally Closed switch at the board terminal or canon plugs, to eliminate one at time that may be contributing to the 450 error. The age of the machine might indicate cable insulation deteriorating. Do some detailed inspections of all the homing and over travel limit switch cables. I have also seen chips accumulate under the way covers that can hold the switch in position.
Power supply failures and/or intermittents can also plague you with similar issues.
The tool changer sounds like it has lost feedback to the control. This can be an encoder or proximity type of switch/sensor. Either the feedback unit is dead, the power supplying it is dead or conductors in cables have finally broken. To hard to say whether these 2 issues are related to the same factors.
It may very well be a simple repair......if you can find it before the men in the clean white coats come to take you away!