Not silly questions at all!
OK, have you checked what weight the airline will allow for DD? They often only allow a further 10kg's (plus buggy) rather than the full amount. Often this is the weight of the car seat which can be a real nightmare. What country are you going to? Some offer the hire of a car seat very reasonably so worth investigating.
High chair, don't worry too much, I'm trying to remember exactly what 16 months was like but you can probably make do. Some fun eating on a rug or something - you're on holiday so you can relax a little.
Sunny climes - uv suit for beach, hat, sun cream and those sun glasses with elastic can be handy (forget their name). If you are beaching jelly shoes or crocs are good for walking on rocks and doesn't matter if they get wet.
On the aeroplane have a few 'new' toys to hold interest but also a few favourites to keep entertained.
The airline should provide you with milk but I would take some extra. You are limited by the liquid that you can take through customs so buy once you are through - they sell all sorts of useful stuff.
Airline food? Don't trust it, make sure you have some snacks, again they sell some good stuff at the airport on the other side. (depending on airport of course).
Hope you have a great time and this has been at least a little helpful.