Khyber looks best in its stock looks,
But if you want to do some extensions then dont overdo it as it will look really bad.
Some ideas what you can do with it.
1) getting sound system installed
Yes that sounds like a decent upgrade.
2) changing seat covers, steering, side mirrors
You can get the steering wheel which came in JDM,s from Sultan ka khu
A memeber Hannan had installed that steering in his Khyber.
Picture attached below.
3) getting central locking installed
Yes thats another nice upgrade.
4) changing headlights and back lights, carpeting and mats
This project will be helpfull in the mats and carpeting section.
For the lights you can get your rear lights custom made with LED,s.
5) changing tyre rims
If planning to go for alloys then get 13" with 165/65/13
6) changing front and back bumpers
Have a look at this.
You will get an idea.
7) modifying the speedometer
You can install leds and it will look like this
You can change the whole speedometer with a tachometer then get a Lexus one.
Procedure and price mentioned in this thread.