Yes the larger the output the greater the price but you still have to look at other factors like the duty cycle. This is the amount of time you can constantly run the generator before you have to stop it for a period of time.
Yes there is, after talking to someone who knows more about generators I found that the voltage of the generator drops when you start getting close to it rated power. Also if you are planing to use a Fridge/Freezer with a generator you must take into account that it draws are on average 3-5 time the current on start-up so there-for 3-5 time the power. So if it was going to be a generator just for fish tanks I would recommended finding one with a long duty cycle and aim to be using around 70% of its maximum output. For a fridge or freezer first you will want to find its serial plate to see what its power demand is, triple that and buy a generator that size. And like anything you get the quality that you pay for.
Buyers guide to generators
Hondas guide to generators