210mm is a nice focal length, and slightly longer than normal (160mm would be about normal on 4x5).
If you want a cheap start you can get an older Symmar 150/5.6 or 180/5.6.
The older Symmar 5.6 can be used without the front element and then you get a longer focal length. The 150mm will also give you 265mm and the Symmar 180/5.6 will double as a 315mm. These lenses are cheap and easy to find. Two lenses in one.
Since lead was banned in lenses these lenses have been recalculated and the modern version is slightly slower at 6.8 (and is not supposed to be used with only one element).