The important thing to realise is how badly you want an engineer to replace Proxy. If you can tolerate Proxy, just buy yourself Nader. (Nader is my favourite merc, currently. Got to love that granade launcher. Quite handy when it comes down to stopping the EV. Not to mention how the SMGs are pretty deadly and accurate.)
But if Proxy needs to be replaced, I would recommend picking Fletcher over Bushwacker. Fletcher’s sticky bombs are pretty similar to Nader’s granades and enable you to help stopping the EV. Not to mention how Fletcher does have some great loadouts to choose from. Additionally the game play will be more varied as you will be switching between your primary/secondary/granades. (Not to mention how throwing granades is pretty silent, so if you can catch the opponents from behind, it is pretty easy to get multiple explosive kills.)
As for Bushwacker… you got the SMGs and turrets. That’s about it. I am not kidding when I say that the turret can help you get a very high kill death ratio, and considerably help defending objectives… but mostly you will be just shooting with a Kek and that can be pretty boring in the long run.
Redeye. Smoke granades are not that great for your teammates without the aid of Vasilli’s heartbeat sensor. Not to mention how most players are smart enough to know to back away from the smoke, or throw some explosives into it. The infra-red goggles are interesting concept. It is easier to snipe at distant targets that glow brightly, but if everyone is fighting in close quarters around the objective, it becomes difficult to actually distinguish friends from foes.
Those are my thougths on these mercs, and Fletcher might be the right merc to pick.
(And anyhow, if you keep playing, you will have enough credits to buy all the mercs. I am nearing level 15 myself and I am only short of Sparks. (Out of the currently released mercs.) )