What spreadsheet apps for the iPad support checkboxes/toggle buttons?
I would really go with numbers, I can say from experience that It will allow you to use check boxes, radio buttons etc.

You would be able to export a Google doc to an excel file and then open in numbers.

If you do choose to export a Google Doc file into an excel file you could also use dropbox to open and edit it in another office based app, with that you may be able to use checks and radio buttons, as long as they already exist in the doc. 

A very good solution is Quick office

You can also try a lot of other free office tools such as Docs Free Mobile Office