There is only one model of MS880 available... with a few bar options. iFcost is not an issue, buy new.
The 084 is hard to find in good shape (and parts...), and the 090.. well, if you like antiques..
Ok, I thought there were model variations such as rev limited coils and such. Re 084 parts, are these saws no longer supported by Stihl or did you mean parts are expensive?