Jasmine, its you who has to decide whether to accept that offer or not?? No doubt, as per current scenario 8500 is quite good for 4 bedrooms but as u have mentioned its 20 yrs old building then its very risky coz the owner might ask you to evict as he has to demolish it and make a new building or villa on it.
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I would recommend that if you can afford 8500 then go for good conditioned appartments/villa [portion]. Search QL,newspapers, NOTICE BOARDS of hypermarkets etc and you might get lucky.
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Rents will decrease more as mentioned in PENINSULA Qatar today.