The mod is meant for max level/experienced players and as such all difficulties are much closer to each other:

• Stat tracking and achievements are disabled, separate save file is used (choose yes when asked to use a backup);
• AI heisters have the same movement speeds;
• Grace period goes down in steps of 0.05, from 0.35 on Easy difficulty to 0.15 on OVK145+;
• Return of Light SWATs and FBI units outside of Heat Street and scripted spawns on OVK145+;
• Easy difficulty uses Hard difficulty enemy health values and spawngroups, Normal and Hard difficulty use Overkill difficulty enemy health values and spawngroups while Overkill difficulty uses OVK145+ difficulty enemy health values and spawngroups;
• Difficulty based weapon delay multipliers are modified to never go down to 0 (only going down to a minimum of 0.25 for Special units (as well as AI heisters), 0.5 for SWAT and FBI units, 1 for Snipers and 2 for Police Officers and Security Guards. This only affects the time needed for units to reach maximum shooting accuracy afaik so they should be more likely to miss their first shots;
• Shotgunners no longer have a reaction time of 0.02 seconds;
• Player armor values per difficulty: 70 on Easy, 60 on Normal and Hard, 50 on Overkill and 145+;
• Automatic player respawns take 3 minutes on Easy difficulty and 5 minutes on Normal difficulty;
• When revived on Overkill difficulty, you get up with less health (50% on 1st down, 25% on every other down);
• Down timer decreases by 10 seconds per down on Normal, Hard and Overkill difficulty;
• Minimum down timer is 1 second on Overkill difficulty and 5 seconds on Hard difficulty;
• The Stryk no longer has ADS zoom similar to other pistols and now has a muzzleflash;
• You now carry 12 Trip Mines;
• Toolkit now reduces interaction times by 40%;
• Cable Guy gives you 20;
• Different matchmaking key so you cannot join unmodded lobbies;
• Added shield piercing capability to the B9-S (code courtesy of Dr. Newbie);
• Fixed Diamond Heist escorts being scared by climbing cops;
• Added AI heister health display on HUD (code courtesy of DorentuZ);
• You can now shoot through AI heisters;
• Added text on top of main menu screen showing that matchmaking is sandboxed;
• Bags can now be shot through;
• Tasers no longer reload your weapon;
• You now carry 2 Ammo/Medic Bags (charges split, Medic Bag has one less use total) and Sentries;
• You can now wear Secret, Troll, Moderator and Developer masks;
• Cloaker now uses the silenced Mark-11 as his weapon;
• Increased NPC weapon variety for Gangsters, Security Guards, FBI and Murkies;
• Static Recoil;
• New weapon overhaul:
• Nerfed Extra Start Out Ammo (+ crew bonus);
• SWATs can no longer rescue hostages (let the FBI do that);
• Increased FBI body HP and Light SWAT HP;
• Cloakers no longer kick the last player standing;
• Sprinting is disabled when carrying Gold (Slaughterhouse);
• Modified FBI/SWAT dodge presets;
• FBI units now use Expert weapon preset;
• All weapons, equipment and crew bonuses are unlocked by default;
• AI heisters now use the Bronco when downed;
• Fixes cases where explosion damage would be applied against enemies multiple times (code courtesy of RedFlame);
• Buffed AI heisters;
• Civilians no longer get scared by gunfire and can survive 1 low damage shot/melee. Does not apply to Bank Manager & Chavez;
• FBI units do more damage with their weapons than other units (similarly to Bulldozers, but filling a glass cannon role instead);
• Added FBI units to Heat Street and Green Bridge;
• Melee can now headshot, bigger weapons deal more melee damage;
• Bundled with fifteen mutators (Heavy Bags, Overcharged Tasers, Friendly Fire, Meth Heads, Limited Arsenal, Bad Trip, No Outlines, Premature Reloads, Disable Auto Reloads, HALFPAY Gang, No Shield Counters, No Crosshairs, FBI's Most Wanted, LMG Dozers and Murkywater Assault (Slaughterhouse only));
• All heists can now be played on Easy difficulty;
• Cloakers drop smoke grenades after downing a player;
• Host can now damage Bulldozer with melee;
• Reworked the Bulldozer so the visor takes more damage than the exposed face (same shots to kill);
• Sentry exploit fix (except Heat Street/Green Bridge atm);
• Murkywater units are harder to intimidate;
• Improved FBI unit category (acrobatic access for all unit variants);
• Changes to NPC weapon damage.

If you are not max level and still want to play the mod, do not worry, as all weapons are fully upgraded by default, as well as all equipment (Wolf Pack no longer gives you extra armor).