About the updating.. I also had that first time I played Gta 4 and just let it update (to 22.214.171.124.) most mods work for that patch too (and Icenhancer always has a beta version for it, which in my experience works just as good).
I don't know how to keep it at patch 126.96.36.199.
Now about Icenhancer, the downloading will take up the most tiime (last Icenhancer I downloaded was more then 4gb if I remember correctly). It's just a matter of replacing files (not installing anything). It's a graphics mod so yes it doesn't include cars (you'll have to download them seperately).
But please backup your game (after you've patched it) because mods in gta 4 replace default things (so you need a backup if you ever want to uninstall something). Icehancer is just a matter of replacing files, if you want to replace cars/weapons/player items you'll need OpenIV or SparkIV. These are programs to open up gta 4 files and replace the right items. I recommend using SparkIV to begin with
If you need any help on how to install those mods let me know, but I'll let you work on the first thing before moving on to the next