firstly welcome to QL and ope u get all the help u need from here..also welcome to doha...
as for ur queries...
1.in 8.5k u might not be able to save much though otherwise u shud manage pretty much since ur utilities and other things r paid for by ur firm....u can get a variety of fones here ranging from qrs 200 (35 quid) to qrs 3000 ( 500 quid)....transportation is relatively cheap compared to many other countries...otherwise the ammount is enough for entertainment movies hanging out clubbing etc...basically provided ur flat,utilities etc r paid for,...8.5 k seems enough for ur personal expenses...

2. even in winters it doesnt get really really cold as it does in U.K so u dont need to carry a lot of warm heavy comforters and quilts...probably a warm blanket might suffice....

3.getting taxi's is a major problem here and local buses aint like the ones in uk so its beter if u have a vehicle of ur own....u will def. be able to get any vehicle ranging from 10k to whatever u can afford...
if ur lucky u might even get a decent vehicle for less than that..

hope this helps...
cheers