If your choice is between the 707 and the 737BJ, choose the latter.
For short haul I would choose the Fokker F28 Fellowship 4000, range 1200. For the little longer routes, go with Tupolev 134 or SE 210 Caravelle 10R. The 737BJ is too expensive compared to the capacity it has, and furthermore its range is only 4306, barely enough to make it across the Atlantic.
If you would like a plane you can afford with that DOP, go for Convair 600-240, high speed (295) and low cost (10m). This will get you going, and when your DOP is above the 5-8 million, go for jets - especially if you aren't extremely active.
When you get the 707, keep it and use it on short routes to big airports (you could of course go long-haul, it's up to you). The 707 is one of the best planes in AM in its lifetime - better than the DC-8.
I hope this advice helps, but always remember to have fun!
EDIT
Just a quick thought: You could try out the VFW-614, it has a high speed (380), and costs just 11m. Sadly, it can only fly for 741 miles, so think it through, maybe you won't have enough good routes to put it on.