It's really not a good time to build a cheap source machine

You need a fairly strong CPU.....the source engine is notorious for hammering the CPU way more than the GPU. With the AM3+ CPUs disappearing and their VFM decreasing, it leaves you in a fairly tight spot. The CPU requirements would make me lean away from FM2, especially if you might add a GPU later. AM3+ is as dead-end as dead-end gets. i3 is not going to meet CS:GO performance expectations and the i5 is a leap in price above all the others.......

So i5 would be my recommendation....at least to avoid disappointment.