I can reply to the first part of your question: these osteosarcoma cells lines (U2-OS, Sa-OS, HOS, SK-ES-1, MNNG, Hs 894, Hs 899, LM7, MG-63, SJSA-1, 143b, and 143.98.2) express HER2:
For SJSA-1, there have been several studies done to evaluate the effectiveness of various drugs (see http://altogenlabs.com/xenograft-models/sarcoma-xenograft/sjsa1-xenograft-model/). There are many osteosarcoma cell lines that should suit your needs, and U20S is a fairly well-known line that there should be xenograft data for.