The easiest way is to give the player the suit at the start and have the 'short logon' flag checked. Then use a point_clientcommand to send the command 'cl_drawhud 0', which will hide the player hud from view. Finally, use the entity nub mentioned (I forget the name, somebody remind me) to disable sprint.
Rather than trying to enable weapon switiching without the HEV, which is a chore and as far as I know needs to be done through code, you can simply trick the player. The only issue with this that I've found is that the player will scroll and still hear the HEV sounds. Naturally if this is unacceptable (playtest it with a few people without telling them what you've done and see if they accept it, don't just dismiss it) you're either going to have to code around, have just one weapon for the player to use, or give them the HEV earlier.
Otherwise, as far as I know, you'll have to get coding!