If the synth works ok,and the keyboard part doesnt then I'd buy it....but only for a cheap price,you should be able to get the spare cheeks from Roland Spares Dept,I've sourced spares for Older Keyboard ok,shouldn't see why the JD parts shouldnt be availible
Coca Cola won't do any real harmful damage,its only sugar constitute and can be easily cleaned off the worst its done will be to form a sticky hard substance which will stop the key contacts from working as in this case,you can clean it up with Isopropyl Alcohol,most circuits can withstand water penetrations providing they are allowed to dry out fully,its the sugar in the coke that will be the most lethal ingredient and can be cleaned up no problem
Easy enough Job to do with care,I repaired a coke damaged Wavestation,then sold it for a nice healthy profit once brought back to life,Infact I've repaired worst spillages on synths,coke is nothing,!!!!try repairing a Polysix with battery leakage!!!!Coke is a good cleaning agent,(thats why I don't drink it due to the chemicals in it)you can remove Oil stains on Tarmac with coke..... the only thing that will stick to the circuits is the sugar nothing more and thats all that will be left if its been spilt inside
I'd pay 100-125 Euro(max) for that JD providing it switches on and works via Midi,its repairable with a little time and patience,the fact it switched on tells you how Little damage the Coke has done to the circuits,and I'd say its affected the Keyboard contacts that all
If it was close to me I'd buy it and do it up and sell it on,easy enough job
Its up to you if you want to buy it and repair it,if the price is right it'll be a doddle,dont pay the guy what he's asking thats way over the top
i mean wood 
