Parts: There should be no need at this point to change or upgrade the parts that ADMIN has set out to use in this $50 robot tutorial. There are, however, better stores to shop at online depending on where you live. I personaly live in Canada! :) Saying that, I noticed that shipping is a little more when you factor in the distance, duties ect, that may occur when buying out of your country. I am going to list a few websites that Should aid you in acquiring the necessary parts for this build.

Digikey -This site offers products in many countries and has fairly reasonable pricing all around.

Robotshop -This site is located in Canada and is a good spot to locate a Sharp Ir Rangefinder, as well as the HS311 servo. (note that you do not need the Sharp Ir Rangefinder for the initial build)

Mouser - This site carries a wide variety of everything you may need for you robot projects. Located in Texas. Have yet to use this site, but im assuming it ships world wide.

Jameco - Another great site to locate and source robotic parts. Also includes tools such as multimeters.

Sparkfun -Yet to be tested by me. Located in Oklahoma. Seems to be another great site to source parts.