Sorry, I missed the 100
kilohertz.
Discones won't give fantastic performance below VHF. A long wire (e.g. an inverted-L) would certinly be a good cheap antenna for the MF and HF ranges. Bearing in mind your comments about neighbors in the general area, it's also worth considering that a long wire is fairly unobtrusive (tell 'em it's a big washing line!
).
You could add an ATU to help tune to the band you're scanning at any one time.