I would suggest using a damping system because ist a bit more realistic and also easier to Work with. Something like this should do the Trick:
velocity.x += max_speed * get_input().x
velocity.z += max_speed * get_input().z
velocity.x *= pow(1.0 - damping, delta * 10)
velocity.z *= pow(1.0 - damping, delta * 10)
This ensures thats the acceleratoon from 0 to 100 ist bigger then from 100 to 200. Of course this method also has some downsights, for example you can't CLEANLY Control between acceleration and deceleration. This method is from
[Jonas Tyroller : 5 Reasons your Indie Platformer sucks] .
It has a lot of useful tips that I also use in my FPS Game. I can only recommend it.
I hope that I could help you. :)