ArchAngel
Hope you enjoy cleaning whole map from the bandits.
Radiation is not bad in small doses. In higher (yellow/red) is deadly.
You either cure them with blue/yellow medkits, vodka (that's why Stalker is cool), antiradiants (expensive) or artifacts like Medusa, but your suit must have at least one slot, and I'm p.sure your leather jacket have none and can't be upgraded to have that slot.
Aim does not improve, Stalker is not RPG. Rifles shoot less accurate because they worn out with every bullet fired.
Guns can be modified, suits too. You can't do it, there must be technician in the base (cog symbol on the minimap) but beware - they cost a lot.
BTW. use crouch+walk combo (Ctrl+shift). This way you crouch even more (don't know how to describe it better) so you can fit in the smaller holes and sometimes reach some stashes, that can't be reached with normal crouch.
Detectors are used to find artifacts in Clear Sky and CoP, the better one - the better artifacts you can find. SGM put option, which make detector depend on the batteries but you can turn it off in the options.
First one - yellow detector - is useless. Start with Bear detector (round display that show direction, where is nearby artifact), then Veles (show exact artifact location) and others introduced in the SGM (Omega, Gamma and else).
Veles can be found in the Swamps, in the Northwest corner (won't spoil how to get there, but it's worth finding, finding artifacts will be way, way easier with him).
Does the game have any RPG like character advancement
No. The only advancement comes from better guns, better suits and right artifacts set.
That's it.
any way to increase carry weight
Better suits, certain artifacts improve it, Hercules pills and exoskeletons.
I am always near or over weight limit
Find some chests or stashes to put all that stuff. NPCs don't steal from it, don't worry.
Bring only most vital items - few medkits (max5) few bandages, something to cure radiation, few grenades... ammunition is important.
If you know there will be fight - get ~150bullets, if not - 70 is max. Shotgun ammo - 40, max 50 because it's heavy. Don't run with boxes of ammo on your back.
Some pistols like Berreta do as much damage as AK74
Which mean you have mod with good weapon pack. Usually they're useless, you need to be veteran of Stalker games if you want to use them effective.