Biggest concern should be the tank itself. It's the most expensive component, and the most expensive to deal with if it goes bad. A bad leak on a tank that size could cause quite a lot of damage. Next would come the filter, last of all the heater.
If you could see the tank up & running it would be best. At very least you want to see the tank full, and not just recently filled. If you see tiny air bubbles, or lines from air bubbles on the inside of the tank, it means it was filled that day. Tell them to leave it filled, & come back to it in a day or two.
Run the filter with water, see if it works. It doesn't have to have media, I wouldn't use the old media anyway. Sometimes impellers on hob filters need a little nudge to get spinning, this is nothing to be concerned with. They may also be a little noisy at first due to a little trapped air. Let the filter run for a good 10 minutes. If it's still noisy, then be concerned.
Heaters are hit or miss, but are one of the cheaper parts to deal with. Some cheap ones last forever, some of the more expensive ones have a bad run & don't last but a couple months. Buy a new one, see if the used one works ok. If it does, you have a spare on hand, which is a good idea anyway.
Tolak