Hi Neil,
I would check to see if everything is working firstly. Skimmer, pumps, heater, lighting. Make sure that the tank has all the parts (skimmer cup, pump nozzles, overflow cup, heater etc).
You should also see how old the bulbs are in the tank, bulbs older than 8 months would need to be replaced in the very near future as old bulbs would not assist in coral growth but will aid in the growth of algae.
You will also want to check if there are scratches on the glass and how deep they are. Apart from being aesthetically unappealing algae (corraline or otherwise) can grow in the scratch groves making it difficult to remove. I wouldn't stress too much if only lightly scratched and it is not in the way.
Ask if the tank is leaking/cracked. If it is then don't buy it, it can be fixed but it is a royal headache depending on the crack/leak.
Very important is to make sure that the stand is level and not swollen. A swollen/uneven stand can cause the tank to crack, something no reefer wants.
One thing to bear in mind is that this is not a very big tank so stocking options are limited. You may have some ideas as to what you want to stock so research the stock in the next two weeks and see if they will be able to live happily in that size aquarium. If your heart is really set on a particular fish and it is better suited for a big tank, is best to hold off buying this.
Another thing you will need is a chiller, don't let anyone tell you otherwise. If the tank comes with it then happy days else this is something that you will need to purchase before adding livestock of any kind.
The main thing when viewing the tank is that you must be happy with the tank and that it meets your needs
Happy reefing!
