starting with mission 5, the game gets way too hard for me.
Are there any cheats like immortal units or unlimited oil?
There is an editor for Kknd 1 which modifies the units google it and search "KKND-Editor Version 2.0 Unit-Editor",
as for modifying the (oil) money can also modify the unit via a memory editor like Art Money.
But it is important to know that the extreme Kknd version that GoG offers prevents the "alt-tab", which is essential for using an editor like Art Money, but if you use Kknd 1 under Dosbox, there's no problem.
In short if you use the Extreme version I can provide you with all my saved game parts (both sides), the backups are only missions where you have to build and create units, and they each start with several million dollars.