1.Is the Mech in Multiplayer?
2.What is bodycount? And more importantly, what are the modes and what they do?
3.Can I map my crouch to a different button?
4.Is multiplayer always so slow when trying to respawn?
5.How do you change your class?

PS:I just rented the game

1. The Mech exoskeleton is not in multiplayer
2. Body Count is team deathmatch.
3. There are 8-ish different controller configurations.
4. The respawn time should always be 8 seconds, unless you get revived.
5. You probably haven't unlocked any new classes yet. You get them by gaining xp and leveling up.

quote dmf_dynasty

1. The Mech exoskeleton is not in multiplayer
2. Body Count is team deathmatch.
3. There are 8-ish different controller configurations.
4. The respawn time should always be 8 seconds, unless you get revived.
5. You probably haven't unlocked any new classes yet. You get them by gaining xp and leveling up.

This is all correct, but can't the host change the respawn time? I'm pretty sure I've been in matches with really short respawn times and some with really long ones.

I don't think you can. I never create my own games, so I wouldn't know, but I don't ever remember different respawn times.

When do I get to unlock medic class, or any classes/weapons? I ranked up twice yesterday, and all I got was being a squad leader and starting a clan

You can find what you unlock per rank somewhere in the Community menu I think. If I remember rightly you unlock the medic at either sergeant or corporal.