Well, the upgreades i'd tell you to buy first:
0: upgreades you really want AND THAT ARE CHEAP, the price is really important
1: cheaper trans I/trans ship capacity (costs only 2k)
2: air trans capacity I (6k)
3: Faster Infantry (MOST IMPORTANT UPGREADE EVER)
4: Cheaper infantry (pretty usefull)
5: air Trans Capacity II (
)
and from this point you'll be rank 6-7 and you'll probably know waht to buy