Your initial concern should not be with the technological aspect of a Bitcoin exchange, but with a legal one. You shouldn’t aim to start a service that would in the end be deemed illegal in the countries you operate, or have servers in. There was a polish exchange that closed down as its owner didn’t want to deal with potential gray areas of local law in regards to running such an exchange.
Then, you should keep in mind the business aspect of things – how will you get money in and out of your exchange and who you will be working with. There have been a couple exchanges like TradeHill that closed because of problems with money flow.
Only having such aspects covered should you deal with the software part of starting an exchange.