The Sparkfun-board seems like a good idea to use for a prototype, but if you intend to get a factory made PCB for your OSD, why use the break out board anyway? It's pretty bulky, and RCA-connectors are not very well suited for FPV use (too big and heavy). There's also very few components on the PCB, so it's pretty easy to add this to your OSD-PCB if you have some SMD experience.
And if you are planning to do this to save
money, I would just stop right now if I were you
Buying e.g. remzibi's OSD would be cheaper in the end.
At last, don't get me wrong. I definitely think you should give it a try. DIY electronics are a lot of fun, so there are tons of reasons why you should
do it, just don't do it for the reasons I've mentioned