Before even considering the switch, you really want to start adding wonders. Right now you have no wonders and I would say that you would want a minimum of 3 before you considered the switch.
Having tech and no Manhattan Project would be a disastrous choice, it puts you in a range where you will be a sever disadvantage; and depletes your money to do so. This is really the key issue.
To a similarly extent however, having a Manhattan Project but not enough money to keep purchasing nukes and fighting can severely hamstring your alliance unless they know that they will be sending you constant aid in a war. Often alliances prioritize 2-4 wonders before MP's for this reason, although its worth checking with your alliance.
Besides that general advice which would put you 3 months or so out, it is always worth paying attention to the political cycle in CN. In a war you should definitely expect to lose a fair amount of tech, so switching to buying 1 month (or even 2) before a war is generally pretty inefficient, as it all disappears instantly and you would have been much better with the extra money from selling. So, if you think a war might come, and are hearing rumors of one, it might make sense to wait until after the next war, and then switch right then, rebuild permitting.