How long are you on vacation for. Underdosing for a week or so probably won't hurt anything, but overdosing.... If you have a lot of stoney corals then ca and alk are of course important. Calcium levels however seem to be slow to change, so I might just focus on alk. If you have an auto topoff system, you might want to mix some buffer with it. If you dose alk and ca together, you will just get percipitate anyway. As you said, you could set up an automated, timed, dosing system (pump), but I think this will run you about $80 per additive.
There are some cheaper drip methods, but you can't dose one at a time and if they fail.... If you choose to do this Kent makes one or you can make your own, I think there have been some posts in the DIY forumn.