Yes the map is being made in GoldSource for Counter-Strike 1.6
Hope you can help 
I hope so too.
First of go to Snarkpit DOWNLOADS or use these links and take a look at
Venko's cool prefabs 1 and 2 [and i mean study it how it's made brush by brush]
http://www.snarkpit.net/index.php?s=downloads&so=1&no=12
http://www.snarkpit.net/index.php?s=downloads
Use them for your map if you want. [maybe re-texture some or something..]
Brush work and texturing that's the key really. You are talking about static cars, not moving vehicles so i don't have to worry explaining how to do that shizm, it's difficult but possible.
You doing it definitely right, it need some amount of practice and skill do make something that looks fairly.Texture placement is pain in the ass sometimes but that's what it is for everyone.You need to build the car at the spot where it stays or else when you move it the textures move too and it is mess again.{ you probably knew it}
You don't even have to use car textures first but those which suits at certain places and look good.[If you want specific cars, textures, you need to make them your self. Look at net for photos or take photos by yourself, edit them in photoshop and with help of Wally put them in wad]
Little simple example:
You got texture of car from side, the texture is rektangle and everything that is not car is blue.
You place it on rectangle brush and get rid of blue parts with assistance of various tools.
Carve
(Carve is when you create brush place it into another brush click right button on it, select carve. It makes hole where the brush was.Like when you want to make quick doorway through wall )
Clipping tool (be careful with these)
Vertex tool (be careful with these they can cause error due to messed up brushes )
Now you have fucking boxy looking car and you start tilt things [you gonna have to separate things so you can tilt them separately] and adding details, mirrors, bumper so on..
It's old engine so you have to be very creative to make things look good.
That's the bottom line.
If something is unclear just ask here
Good Luck mate
Let's Rock'n'Roll