I am looking to buy an easy to assemble medium sized knock down bird cage online. Can anyone recommend a good option?

I recommend getting at least one medium-sized natural wood perch, one rope perch (again medium size), and one other type of perch - maybe a therapeutic perch or a second natural wood perch. Here are some good examples for your Parakeet.

http://www.amazon.com/dp/B00106TFJU?tag=allpetbirds-20&camp=213381&creative=390973&linkCode=as4&creativeASIN=B00106TFJU&adid=0NHYMRNQYM72AG285TXA&&ref-refURL=http%3A%2F%2Fwww.all-pet-birds.com%2Fparrot-perches-medium.html

http://www.amazon.com/dp/B0002AR77M?tag=allpetbirds-20&camp=213381&creative=390973&linkCode=as4&creativeASIN=B0002AR77M&adid=084B1Y3HYVZ1BAJ5DDEQ&&ref-refURL=http%3A%2F%2Fwww.all-pet-birds.com%2Fparrot-perches-medium.html

There are plenty of options to choose from. I will attach a few links to get you started, but I recommend doing a little shopping around of your own.

http://cozzzybird.com/

http://www.bird-supplies.net/dometop-covers.htm

http://www.birdcagesgalore.com/bird_Cage_Covers/Dome%20Top%20Covers%20cozzy%20cover.html

As long as that is your minimum size, you should not have a problem finding one. The only issue will now be price. Here are a few examples to get you started in your search.

http://www.bird-supplies.net/nina-dometop.htm

http://www.birdscomfort.com/cage_nina_dometop.html

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=14410

Even though budgies are quite small, they are very active and need space to fly around and exercise. Budgies need a minimum cage size of 18"x18"x24". Here is a sizing chart that should answer any questions regarding bird species and cage size, along with particular suggested brands of cages to buy.

http://www.bird-supplies.net/size-chart.htm

I suggest you to have a look at http://www.birdcages.com/helpfulinfo.cfm. I personally purchased a parrot perches from their site and I was very pleased with their customer service and their products. If you're looking for a bird cage you'll find it there.