Arieh,
I think your requirement is still not clear. Let me take an Example and mention what is missing in that. Check below example and mention what is missing. Though you original question was whether you should go for multiple currencies or not? I hope that has been answered, if that's the case close one topic and open another so that things they don't get mix-up.
| 01,2015 | 02,2015 | 03,2015 | ,,,,,,, | 01,2016 | ||
| Sales Forecast | PC | 10000 | 15000 | 12000 | ||
| Avg Selling Price | EURO | 2 | 2 | 2 | ||
| Sales Revenue | USD | 26000 | 39000 | 28800 | ||
| Conversion Factor | 1Euro to USD | 1,3 | 1,3 | 1,2 |
Regards
Rahul