I would look for an umbrella style one so that it folds quite small but make sure it has a good recline in it and a large hood/sunshade. I have a maclaren Techno XT and it is great. I used it for my DD since she was just over 1 and I took it on our last trip and she was 4 and she was still comfy in it.
I second a vote for the maclaren buggy I have the Quest buggy and its fab for dd whos nearly 3 and ds whos 6 still fits in it whenhe needs to rest his legs.The maclarens are expensive but there are usually loads on ebay if you want to pick one up cheap.
Hiya
I got a Mamma's and Pappa's Ziko, my ds is also 18months and there is plenty of room, and i also got a free matching parasol,
So my vote would be for one of those, saying that my mum had got a McClaren for when she has got one of the grandkids and she loves hers,
Ali
If the buggy is not needed on the flight or when you are back at home, I would buy one at Wal-Mart and either dump it before leaving or give it to someone in the stroller hire queues at a park. They are way cheaper over there and this is what we did for the two trips DS needed a buggy just for the trip. Much less hassle and cheaper!!
Hugs
Joanne