I say this as a daily driver of a stock P38 for 4 years that also weekend 4x4's out by Rainier and the Cascades...
If this is your first P38 talking of modifying the suspension is something that should wait until you better understand the vehicle, and that point I don't think you'll feel it necessary. It would be better to change all fluids, including the gear oil in the diffs, replace
all the yellow and green relays under the hood, the circuit board too if you can swing it, and the fuel pump. Also the battery / alternator need to be good. Don't assume - you need a baseline. Do that and you've sidestepped most of what people on here complain about.

I can provide you cheap US links if you want them.
32" BFG All Terrain KO2 or Hankook All Terrains and a stock P38 will ford rivers and cross mountains.