Having one rebuy is ok, having two means less stress. And it's not that difficult to secure enough credits for the rebuys. For example - iCourier bouncing between Sothis and Ceos for an hour, delivering data = ~22M.
What's more, you stated you'll fly in Solo - I don't see much danger from that. You can win interdiction attempt in Anaconda easily, so no threat from here. Passenger missions... maybe when wanted people and being scanned at station. Anaconda isn't the best one for fast mailslotting. But again, there are other tricks to avoid scans.
OP, do not worry much, with your current build set that 18M as a bottom line and you're set.
What's more, you stated you'll fly in Solo - I don't see much danger from that. You can win interdiction attempt in Anaconda easily, so no threat from here. Passenger missions... maybe when wanted people and being scanned at station. Anaconda isn't the best one for fast mailslotting. But again, there are other tricks to avoid scans.
OP, do not worry much, with your current build set that 18M as a bottom line and you're set.