Cars are kinda tricky indeed, there's this bug where if you install custom cars you see alot of taxi's and police cars. To get those custom cars it's better to install simple native trainer (to spawn the cars) because you'll have a hard time finding them ingame.
As for graphics mods I'm always refering to this one: http://www.gta4-mods.com/misc/icenha...phic-mod-f9503
I have it myself and it works excellent, it's still the 2nd most downloaded graphics mod on that website
I'd also try to install a gun, doesn't matter which one just to confirm everything with importing etc is working properly