Available equipment and credits are dependant on your level. I played through the game twice in sucession, obtaining level ... 57, I think?
Anyways, my point is you pick up a lot of the decent stuff you need. Credits buy you the odd nice upgrade but to be honest you'll get most of your items from pickups.
By the time I hit lvl 30ish, I had had a million credits in the bank simply by selling excess gear off and surveying/raiding planets for everything I could find. Doing all the side missions proved great for gaining more credits.
Top Tip: Make sure you open every receptacle you can, even if you end up using Omni-Gel to open it. If you cant use, or its pointless or doesnt upgrade what you have, then you can sell. Explore!
When you get a million credits (in total, not in the bank) you can access the lvl 7 Spectre weaponry on the Normandy or from the C-Sec seller. If you max out Spectre Training, yo get access to Lvl 10 Spectre weaponry, which combined with Frictionless materials makes you pretty unstoppable in a fight
Lvl 10 Spectre Sniper rifle + Assassination + Upgrades = Dead Geth Armature in 2 shots
Edit: Top Tip 2: Stick with just two of the available allies, just make sure its not both Alenko or Williams at the same time. as an Infiltrator, I used Garrus and Liara 100%, so only had 3 characters to upgrade.