The numbers need to be printed with Micr so the bank can read the information in their automated systems. This can be done offset or some digital presses can run micr toner. So if you are buying blank checks with the security features such as thermochromatic ink and watermarks already on them then you need to print the name and address, routing information and check number. This can be done on an AB Dick press with a numbering system attached to the back of it to stamp the consecutive check numbers.
I'm not sure it's the business to get in if you are not doing it today as I don't see this a growing area, just thinking about how few checks I write in a year with online payments becoming more common. Deluxe Corp who has some diversification but still about 75% checks had a 2% decline overall in their business I would expect that to continue or get worse.
I outsource mine, the micr ink is very expensive and can be a pain in the neck to clean up, try your local forms house, ennis is who I use and they are every where - make sure to check around - these guys aren't the cheapest but they are close to me for local pickup so it saves me the extra in shipping. Good Luck - I hope you have a lot of checks to print if your seriously looking into it. You might try a good OKE data printer if you just want to micr a few. Still expensive though.