Difficulty versus Reward
There is no point in playing gold because you get much less credits if you can’t finish it reliably. It usually takes much longer than silver and bronze anyway which negates the higher reward. I find that I need very good teammates to finish the gold challenges and at least decent teammates to finish silver.
Generally the best challenge to choose is the challenge that you can complete reliably. If you find that you usually fail the silver challenge just play on the bronze challenge. However, the silver challenge is the best reward for the time spent and the difficulty level. You get around 30 000 credits. The bronze awards you with around 15 000 depending on how fast you completed objectives.
You can buy packs using bioware points, but a spectra pack comes down to costing around $2 which seems a little much to me. It also takes away from the game if you just have everything you want instantly.
Best Class to Use
You want to use the class that you are best with. If you are farming credits don’t bother with leveling a low level character because you will just get frustrated. I wrote a guide to the best class to carry your team, you can find that here.
Best Upgrade Pack to Choose
You can choose from a 5 000, 20 000 and 60 000 credit pack. It seems as though the 5 000 credit pack is the best, but in the more expensive packs you get much more. So in the basic you might get one medi gel revive, but in the spectre pack you get 5 or 6 of them. Also the chances of getting good stuff are much higher and guaranteed to some extend in the spectre pack. I have gotten as much as 3 gold items in one pack before.