Figure 50-60mm for the stem, and some for the headset, which doesn't have that much stack since there's no locknut and a 300mm fork will be fine if you go threadless. It'll also have enough room for your cable hanger.

You could also buy a steel threadless fork and have it cut and threaded. Figure that will add about $25-35 to the cost, depending on who does the work. You may get the best deal on labor if you let the seller do the work and quote a combined price. (otherwise I do this routinely for $25.00, plus return shipping).

Note that not all threadless forks hace the correct 7/8" ID for your quill stem, so confirm this if you buy planning to thread.

If you can't find a decent fork on your own, call Yellow Jersey in Madison WI, for a quote of either threaded or threadless version. They have a broad inventory of forks and might have a 260mm threaded fork, and/or can cut/thread if that's the route you want to go.