How frugal does one have to be with credits in the game? (NO SPOILERS PLEASE) ALSO. Favorite weapon mod/armor set?

I want to try a lot of the guns out. I could easily buy one of every starting level for a specific weapon but I wouldn't get a feel of how much the upgraded version would do. I don't want to waste credits. I know in ME2 they were very hard to come by.

I am still pretty early in the game but what armor is good to use?