This won't work.
Don't know how reliable they are, but according to
totalsportek Ozil is currently earning £140,000/week in Arsenal, while Arda's wages are "only" £85,000 weekly.
(same source)
Now he apparently wants more than £300k and if that's true then we shouldn't even bother trying to negotiate with him and Arsenal.
If we can really get him for around 20M my maximum offer would be a 2,5 year contract (until 2020) - possibly with an option for another year if he plays let's say 60% of the games for at least 45 or 60 minutes in season 2019-20 (that way we'd know he'll be at least useful squad player if not a starter) - in the range of £170-200k (so similar to Busquets and Iniesta - I don't think he deserves that but whichever big name midfielder we'll buy we'll have to pay him at least that much) with bonuses for individual and team achievements like other players have (don't know exact numbers). Take it or leave it.