18 should be ok. May actually be a little warm once things get going. Fermentation will create its own heat and the temp in the beer can be much warmer than ambient, like 5 degrees, sometimes more.
I would also recommend not trying to hold to the timeline in the instructions. It will be done when it is done. I would wait at least a week, maybe even two, before checking the gravity the first time, then again a few days later. A little extra time is better than not enough. If it is stable I would go straight into the bottles, no secondary. Assuming nothing is being added and this being your first brew, secondary isn't worth the extra risk of infection.