Car modules not operating

I recently bought the vehicle used from the dealership with about 107,500 miles. From the get go, the car was having a slew of problems, horn doesn't work, A/C fails, and loss of brake pressure when I have to slam the brakes. I bring it in to the Ford dealer to get it diagnosed and they say the modules aren't working properly and are charging about $300.00 to $700.00 to replace each one (A/C module, steering column module, ABS module) for a grand total of about $1,500.00. I take it to another mechanic for a second opinion and he gets the same diagnosis, however his recommendation is to replace the cars main computer which should in turn fix all the cars modules. The second mechanic is charging around $200.00 to do that, the huge gap in price between the Ford dealer and second mechanic makes me kinda iffy and don't know which recommendation I should move forward with. Anyone have experience with this and can provide a recommendation?