Spruce Box mast
The boat that we just bought has a spruce box mast on it, so I just went through the research you are doing. Generally, I found that spruce masts were well regarded strength-wise. They do weigh a bit more than aluminum, but one source indicated that the extra weight aloft actually made the roll of the boat more comfortable. On some older, short-masted gaff-riggers they used to haul anchors aloft during storms to reduce the roll...
Anyway, the thing that I would really be concerned about is the rot. We thought that our mast was in pretty good shape--the owner had just had the boat partially re-rigged 3 years ago and had kept up the mast painting since then. Well, during survey we discovered that the mast had such significant rot at the gooseneck that the rigger would not even go aloft to examine the rest of it. The rigger told me that my experience was typical--he finds rot in almost every wood mast he examines.
If you are looking to buy this boat, I would get an experienced rigger to go aloft and check the whole spar. Examine every pentration and pay particular attention to the masthead. In our case, we found that we needed a whole new mast--and the next one will be aluminum!