1) Only actual forces count. Ammo doesn't.
2) Activated forces have a higher upkeep, use more officers/soldiers, and have a higher effect than deactivated forces (I believe it's about a factor of 4 for all of these).
3) Defense Index = Defensive Force Index * 50% + Offensive Force Index * 25% + Strategic Force Index * 25%.
4) Given that Defensive Force Index has twice the effect of the other indices on your Defense Index, and it's the cheapest to raise, if it's the sole intention to level, then MIB's are the cheapest/most effective method of doing so.
5) The Defense index moves quite slowly. If you decide to just buy 60k of MIBs over 2 - 3 game months, except quite a lag before the index catches up to the reality of your holdings.
6) This one isn't specifically to you, but to other new people who may not know it. In leveling, it's the average across all your countries, so if you're going for Level 10, as an example, you'll need an average Defense Index of 110 across all your countries, not just the main country. This holds for all the other requirements, as well.
If I remember the numbers right, it's around 62k of activated MIBs to get a Defense Index of 100 (assuming all the contribution is from the MIBs).
-Daelin