I have no idea really (newb!) but I'd go the flattener, then later if you feel the need the reducer. To mine, there are very few targets that need less than 500mm. And for the ones that do, mosaic
The least expensive question you can answer yourself: take a shortish (30s?) exposure of a reasonable starfield and look at the star shapes in corners and centre. Stars will be elongated in the corners and tight in the centre if you need a flattener. Please post your results I'm keen to learn whatever I can
