The heat issue that you are referring to is when you use a 7,200 RPM hard drive in the WJ-HD500 or WJ-HD500A unit. The WJ-HD500B unit was redesigned with fans for each hard drive to support 7,200 RPM drives. If you have a non B unit, just make sure that you use 5,400 RPM drive, and do not store the unit in a lock box.
The WJ-HD500 is a great unit that holds its value. It is still a hot item, and last a long time.
Remember that with any DVR, the hard drives are considered consumables, and will need to be replaced eventually. Some places with high use, and lots of motion detection may only get 1 year of live per hard drive, other places will last 3 years or more.