What router is best suited for a small internet startup?

If you're well versed and comfortable with Linux, etc. you should seriously give Vyatta a look (www.vyatta.com). They have both a community edition and a commercial edition. You can strap these on your own hardware, or alternatively, buy the hardware from them.

CLI configuration is very much like a mix of Cisco's OS and Juniper. Very reliable and mature product I might add.