I really have a keen eye for the Caravelle. From the IVC onwards, iirc, it requires only 2 pilots, which is very, very rare for the era and a must in high labor cost countries. For airports that don't have a huge domestic demand, they really suit well. Plus, the cargo version will be amongst the smaller in the large category, making it a potential killer to the 727F.
Otherwise, as the other said. The Viscount is pretty good and can last very long (~early 90s, but if you do this you need an exit plan).
The Tu-104B (but not the Tu-104, beware) is not bad at all given you don't operate it too long. Will be dirt cheap and easy to get, and this is what makes it interesting. Surely not a state of the art aircraft, but if you're in trouble and can't get your hands on anything else, it's definitely worth considering.
You then ask about what time you should move on from props to jets. It really depends on how you want to play it - potentially you could keep some Lockheed Electra a tad late, or got right away with the Tu-104B or Caravelle, which are amongst the earliest jets.
Basically, it will depend on:
- what airport you set your HQ in
- what strategy you want to develop (Detroit would force you to dig in more than St Louis or such)
- what's available on the UM at game start might influence your fleet building for ~10 years, even if you didn't thought it this way beforehand.
So start with an open mind, but also determination
