Hey OP, for your first account, you want to make sure to have in game consumables to use every match. This will help you complete matches, meaning you are maximizing your credit gains.
As such, strategically buy JEPs when required, as these will provide good value with useful consumables. This is very important.
Early on, you (like everyone) will want to unlock more powerful weapons and characters. Save for 99K packs regularly (mixing in the purchase of JEPs to resupply) as these will provide the most guaranteed rare cards (and by extension, better odds of getting ultra rares)
It sounds like you are past the very beginning, where I would recommend spending the first 40k or so on recruit packs to upgrade your common items and acquire a base of consumables.
IMO, one of the biggest fallacies other posers will state "buy veteran packs until your uncommons are maxed out". Your manifest is about strategic balance between always having enough consumables and adding / upgrading your weapons / characters.
The last thing you want is few to no rare guns or characters, yet 200 rockets and med gels. This makes no sense as the credits used to amass that many in game consumables could have been used to acquire more rare cards faster, providing you with more playable options.
Once you have most or all the rare items unlocked, consider starting to buy Spectre packs, as these are the best value for a more develloped manifest. They will mostly supply you with a good base of level 3 consumables to use on higher difficulties. And remember, throughout your time playing ME3MP, always buy startegic JEPs to refill your consumables .. so important.