Anyone know what be good iron wattage for cap changes?.
This kind of depends more on what the cap is attached to, rather than the cap itself. A low power iron of 15 Watts will work fine for most repairs but if you get a component that is connected to a large ground plane or power buss it can often take a bit more power. A little extra power is also sometimes helpful if you are trying to desolder with solder wick.
My advice would be to get an Iron that has a variable temperature control and a maximum wattage of 40 to 60 watts. I know 40 to 60 watts sounds like a lot but with the variable control you can turn it way down when needed. I think you'll find that a variable temperature Iron won't cost all that much more than fixed temperature iron.
Regards,
AmigaEd