You will not be very happy long term with the Green and below mechs.
As soon as you save up enough to get a blue one or epic one. Focus on it and save all your resources for it and it's weapons. At some point you will start to have some surplus reaources to spend on others. One badass mech with take you farther than 5 okay ones.
You will need 5 eventually but be patient at first. You will be tempted to level up everything. Weapons and mechs require a lot of A coins to Rank up and you won't have a lot of them so save them for your special mech.
Also, power 8 and below weapons don't deserve your A coins in my opinion. I use credits to get them to level 7 and leave them at the rank I obtained them. Shotgun 8 was an exception but with the new shotguns coming don't waste a ton of resources on them. When you get 10,12 and 16 power weapons you will need lots of A coins.
Have a immeasurable amount of patience and play for fun with friends. Makes the taste of some encounters less bitter.