When I needed a 110 to 220 transformer I also needed more functions so......
What I have has 110/220 in, 110/220 out + isolation.
I use the isolation more than anything. Some transformers do not have isolation.
Also many transformers have a way to get slightly more and slightly less than normal voltages. When I test power supplies I want to see that they not only work at 110 but 'low line' and 'high line' 110 and 220.
Your idea of using 80 watts from a 500 watt transformer is good if you can afford it. Using 80 watts from a 100 watt transformer will cause the voltage to sag some. The small transformers are 'soft'. Many transformers, when run at full power, will be over 100C. HOT