This is assuming you have a lathe and other woodworking stuff like saws, drills, drill press.....
1)A great chuck like an axminster or oneway with multipe jaws and a jumbo jaw. ($600)
2)A sharpening center. A grinder with a wolverine jig setup including a fingernail gouge jig. ($250)
3)A set of good HSS tools for bowls including a good fingernail gouge and a set of smaller tools. ($350)
4)Sawsdust collection system ($150-?)
5) Pen kits, multiple mandrels, top notch drill bits, pen mill, organizing bins, display boxes, felt pen roll, business cards, sandpaper, CA, micro mesh, and assorted cool pen blanks.....spend all that's left.