2008 Chevrolet Express transmission running hot

Just purchased Chevy express 3500 one ton van with 240,000 miles North Carolina car, very clean. No service records. Driving back to Michigan after about 400 highway miles, message on dash "Transmission running hot, Idle engine." Van never bogged down or had any trans slippage or burning smell. Engine did stall when shifting in reverse a couple of times. Pulled over and turned off engine for a few minutes, started engine and message cleared. Shop diagnostic ran two codes leading to possible sensor malfunction and/or transmission overhaul. On inspection trans oil is clean and not dark. Shop suggests changing trans fluid with new filter, install new sensor and an external trans radiator to bypass original radiator which might have sediment blockage. Is this advisable? Any advice would be welcome.

Dave