Your airline is Rusair, correct?
If I were you, I would focus on large boeing & douglas fleets becuase they are often easier to replace later (especially if you use them for less then 3000-mile long flights) as you can easily replace them with something like the 737-400 without too much fuss.
I would not reccomend going for the Caravelles as they have a bit too short range to do the Moscow routes but are a bit too big for the small Siberian routes. For the 0-700 mile flights I would reccomend the fokker F-27. They have served me well in other worlds, also they sold well in the real world (if you want to maintain some realism). Also, if you want to fly less-than 5 daily pax routes the DH114 heron has served me quite well in my airline (Cambrian Airways) for longer flights such as southern England to Shetland islands.
In terms of short range, mid range and long range. I would probably say 0-1000 miles is short range, 1000-3000 is mid range and anything more than 3000-4000 is long range. Also, if u need to replace your DC 7 eventually I'd go for the Il18 (they are dirt-cheap, and while they aren't always super profitable they have a good range and can connect moscow and sain petersburg to siberia. Also, they are russian so they might be a more realistic option