Skeletron's seem to turn up often enough, I've got about 10 now. The most I paid was £10 for a really mint one, but also paid £2 for a few mint ones too! I personally like the skeletron, so always keep a look out.
As for Muton, I've toyed with the idea of customising different weapon attachments for his head, but for a handheld weapon, I'd be tempted to go with something brutal like a sledgehammer or a mace!
I've found that Muton holds most of the A.F. weapons well, and in some cases better than the the figures the weapons were issued with..the a.k.47 fits perfectly, so that's what my one's normally tote..