The manual really tells you about most of what you want to know. It isn't entirely correct, though. Contrary to what it states, there are no upgrades for the "sniper rifle" (SLNT-something). The HAWK-MK8 on the other hand can only be upgraded through a specific quest, whereas the flamethrower can be upgraded just as usual by buying the upgrades.
Unfortunately, many of the "gadgets" are not very useful. Invisibility (PPS) is good to get out of trouble and to plant explosives in groups of enemies without them noticing you, though you have to quickly detonate them with the CLAPR-T before they run away. I didn't find any other use for that, as you can only use items and no weapons while invisible.
The UBIK-something creates a hologram of yourself, which can be used to distract enemies, who will try to shoot it. I never had a use for that, as I think dodging enemy fire is easy, anyway, but you can use it in larger battles to avoid the enemies' gunfire.
The PROXI-130 (or so) shows an arrow around yourself which directs you towards the closest living being, usually enemies in fights. I thought that was just useless, since you can more easily see them on the minimap if you need to.
Lastly, the "thunderstep" is a proximity detector. You can put it on the ground, it will be activated as soon as it starts blinking, and when an enemy comes close to it, it automatically detonates all explosives close to it. This can be used to easily lay traps, for example to enemy patrols, or when enemies are running after you around a corner.
All in all, I think Outcast is just too easy for any of the items or weapon upgrades to be really very useful. But, I still liked using most of them just for the fun of it. Sure, I could just gun down any enemy with the Level 0 pistol, but I still ambushed enemy patrols by laying explosives on the ground and detonating them from cover when they came close, tranquilizing them from far away with the sniper rifle, or just annihilating them by raining down grenades on them :D
So, I recommend that you don't save up too much on money, ammo and the items, just use whatever you think is the most fun in any situation. You will have more money than you can spend at the end, more items than you could use up, and by far more ammo than you'd need to eliminate Fae Rhan's whole army, while you'll still aways have several ways of getting more of everything :)
Just one last hint: Don't forget that you can always change the weapons' levels in the inventory (Press I and click on the weapon icon to open it). For example the grenades of the LN-DUO grenade launcher behave differently with each upgrade, but you can always switch them around. Thus, you don't need to feel bad about buying an upgrade you might not have much use for.
Post edited May 17, 2010 by Syrion