1. It's a bit hard in the beginning, but not too hard. You have 3 difficulty settings in the beginning. Start at easy or normal.

2/3. It's not necessary. You're new, 1.13 will make it a lot harder for you. Get familiair with the game first. Once you know it, install 1.13 as it adds a lot like some bugfixes, new weapons, alternative inventory system, etc.

4. There is a mode to make in even harder. And there's the default and infamous Drassen attack. Once you've taken the whole of Drassen you get a whole lot of enemies thrown at you.

Some starting tips:
- money is an issue at the start, so don't waste it all on mercs
- don't waste time in the beginning
- have the basics covered with your first mercs, thus explosives, repair, markmanship
- when in battle always have your mercs cover eachother, looking at all angles and when possible make sure they are in hiding
- rest when necessary
- don't play in sci-fi mode.
- talk to all named npc
- put all money, gold and silver on your bank account (click the $ button)
- hire Ira in Omerta
- always explore a sector, you can find stuff on enemy bodies and in closets, medicine cabinets in toilets, etc.
- some items like a metal tube, glue and tape can be combined to a gun barrel extender. Apply it to a gun using a high repair merc
- when you have silencers, sniper or laser scopes, bipods or grenade launchers, attach them to weapons
- armor your mercs
- visit at least IMP, MERC, AIM and Bobby Ray's website
- let mercs use only those weapons they're good at. No need to have a burster use a sniper rifle or vice versa

My prefered starting team.

To skip combat in sectors you want to cross, you can do the following:
- put your team in the corner
- leave the sector to go back to travel mode
- go right back and pop up at the desired side you want to leave the sector from

After entering Omerta:
- clear the sector
- find Fatima and follow her
- take the quest to bring a message to the priest
- hire Ira (she is the one needing to talk to the priest) You can train her into anything due to her wisdom
- clear Drassen

Once you've taken a sector, try to train militia.
If you can't, take the following sector and try again.
Use mercs with high leadership to train militia. In Drassen, use your custom merc and Ira (she learns fast due to high wisdom).
Once you've conquered Drassen (pliers are useful when entering the airfield from the north or west) talk to the mine foreman using a merc with high leadership. He will work for you and the mining income will be yours.
Income of a mine depends on town loyalty.
You can sell weapons to Tony in San Mona (visit it after Drassen).
You can do several subquests which will get you more loyalty in a town or some extra cash.
Be sure to take the SAM-sites as use of the chopper makes traveling a lot faster.

If you have any questions, just ask.
Once you're hooked on this game, you will go back to it over and over again.
Be sure to take a look at the Bear's Pit too once you've familiairized yourself a bit with the game.