Don't stress. It will be fine.
Check with the airline re buggies and allowance.
We've only done long haul with little people (to see DH's family).
The buggy was tagged like a suitcase at check in, but then we kept it til we boarded. Sometimes we got it back at the aeroplane door. One horrible flight, we ended up going all the way to baggage claim....
The baby also got a half decent baggage allowance (maybe 10kg?) plus the buggy.
Milk - yes, it will be fine. You (or child) may have to taste/drink some of it.
You can also usually buy milk air-side.
DS1 was so excited and enthralled by it all, he was 
for the first couple of hours.
Consider putting some toddler apps and vidoes on your phone / tablet if you have one.
Also consider time consuming snacks - raisins, cheerios, satsuma to attempt to peal and eat? And some sticker books - amazing value for little people, and yes, at 18 months they can get it.
Pack extra nappies and clothes for toddler in hand luggage. Also consider a clean top for you.
When the cabin crew thought I was travelling alone with 2 small people they were really helpful. They were still helpful when they realised DH was just the other side of the aisle. Don't be afraid to ask for help or food / drink at non standard time (if your not flying budget!)
And however loud the screeching sounds to you, aeroplanes are noisy places, and a very few other people will hear it.
Hope everything is OK at home 