Are you asking if you should upgrade your radar or purchase the C&D SR first? Either one would work. You can purchase C&D to earn more blocks for upgrading, or upgrade to earn more blocks for purchasing.
Upgrading to LV.7 needs 2500 blocks, but the boost rate only increases by 0.5. If you wish to see a noticeable change in the blocks you earn, you should at least get your radar to LV.8, which grants you a 2.5 boost rate. That would need another 5000 blocks, so a total of 7500 blocks are needed. Will you be able to cover the cost? That depends on the average blocks you can earn per day. If you want to upgrade, please do it fast as the event is nearing its end.