Hint #1: How to unlock level select option? You have to edit the "hitmancontracts.ini" file that you can easily find in the game folder. Then, find the following line:
"DefaultScene=AllLevels\\Logos.gms"
and replace it by
"DefaultScene=AllLevels\\LevelMenu.gms".
Hint #2: How to get a second chance? All you have to do is to shoot 4 people (try to shoot them in their heads) when the screen turns black and white. Then, resume the game with your health refilled.
Hint #3: How to unlock all weapons? While in training level, enable the "end level" code. Thus, you will have a chance to complete the level with a "Silent Assassin" rank. Then, you can choose the training level again.
Hint #4: How to get bonus weapons? You should complete the indicated mission with a "Silent Assassin" rank:
CZ 2000 Dual Pistols: Mission 1
Micro Uzi Dual Submachine Guns: Mission 2
Sawed-Off Shotgun Dual: Mission 5
M4 Carbine Silenced Assault Rifle: Mission 6
SG220 .S Dual Pistols: Mission 7
GK 17 Dual Pistols: Mission 10
Micro Uzi Silenced Dual Submachine Guns: Mission 11
MP5 Silenced Submachine Gun: Mission 8
AK 74 Silenced Assault Rifle: Mission 9
Silverballer Silenced Dual Pistols: Mission 3
Magnum 500 Dual Pistols: Mission 4
PGM Silenced Sniper Rifle: Mission 12