If the trouble started after Best Buy installed the remote start feature I think I'd be talking to Best Buy next. $850.00 sounds steep to me but Lexus is proud of their work. You could pick up a used fuse block cheap and replace it yourself.
The rotors fit snug over the hub. Spray with PB Blaster of WD40 and let it sit for awhile. You'll probably have to persuade the rotor off the hub with a big hammer. Read the following: Rear rotor
I did a lot of research before I bought Mohawk Predators. I went with IS300 17" wheels and wanted the biggest tire I could put on it. I found 245/45-17s. The ride is nice and quiet. Mohawk is a subsidiary of Yokohama. I got the wheels and tires for just under $1,000.00.
I was losing coolant and after changing radiator, hoses, and thermostat it proved to be a weak radiator cap. I agree, get it pressure tested to be sure.
I had a set of chrome oem wheels I sold to a "fellow" forum member here. I miscalculated the shipping and sent them anyway with a note to send me the difference. I guess he left the country. Got stiffed. Live and learn. Check Ebay.
I had a similar problem. Replaced the hoses, thermostat and radiator. Was still losing coolant! Changed the radiator cap and all is well. Think about how many cycles the cap goes through. Wish I would have thought of that first.
I think his asking price is too high. Check KBB.com and Ebay for similar vehicles. Why would an aftermarket head unit cause the power locks to fail? Start low and he should come down.
DTC 54 trouble areas: TRAC Actuator
TRAC Accumulator
Brake fluid leakage
ECU
I'd check the simple things first. Connectors clean and tight and so on. You need to get the Lexus manuals to trouble shoot. ABS and TRAC control are inter related. Check and make sure the ABS sensors on each hub are clean.