First off.. if you install new cars you should always try to start with a new game, parked cars that have been modified after the savegame are usually causing issues when you load that savegame again. That could be the cause of your game freezing up
The disappearing textures could have various reasons, do you have trafficload.ini in your main folder? It's an anti-taxibug but can sometimes be too heavy on your pc and cause texture problems (no matter how good your pc is)
I'd indeed recommend a clean install and taking it slow, don't install too many mods at once (so you can see which ones cause the most problems) and try to go easy on the car mods. In my experience you can install as many player and weapon mods as you want but car mods can do damage to performance if you install too much of them along with a taxibug fix.
Personally I'd refrain from installing those HD textures, they look nice but the game runs alot smoother without them