I say go for it! Companies like Small Wonder Labs have been around for years and I'm sure will be around for many years to come. Kit building is alive and well, especially in "this economy" where people are willing to trade some time building a kit to save some cash.
Sure, some components are cheaper/only available as SMT. In those cases, use the SMT part. Thru-hole resistors and capacitors should not be a problem to find cheaply.
As far as soldering SMT goes, take a look at the Schmart Board system. It allows a 9 year old to accurately solder all types of SMT parts. http://www.schmartboard.com/index.asp
Perhaps you could partner with them to use their technology in your product.
As far as ideas for what to build, I have built all kinds of QRP CW radios, PSK-31 transceivers, etc. One thing I have always wanted to build is a 100w HF transceiver for CW and SSB. Something simple and to the point like my HW-101.
Make something simple and reliable that works as advertised and it will sell.