SOPS is correct, you won't be able to live with your fiancé in Doha until you get married because you can only be her sponsor if she is your wife, so there's no way she get can a residency permit without being married to you (unless she finds a job of her own and the employer sponsors her).
Regarding company accomodation, yes, they will send to company accomodation initially. As people have said above, they vary from bad to decent/good. You can change accomodation once you are in Doha (it might take a few months) or ask for the housing allowance (which also might take months/years depending on company needs).