Heater

The recommendation is 1 watt per litre. So for 80 litres you need 80 watts. Of course no-one makes an 80 watt heater so you need to go up to a size that is made, and the most likely one you'll find will be 100 watts.

Heaters are something you need to be sure about. It is not worth compromising quality to get something cheap. You can get away with cheap filters but if something goes wrong with a heater you could end up boiling the fish if it gets stuck in the on position.
Personally, I like Eheim-Jager heaters, the only downside to them is that they are slightly wider than most makes so it is almost impossible to buy a heater guard for them.

Fluval is also a good brand so that would be fine. It's the Chinese imports with odd names you need to be careful of.

Thanks