i think you should bring it to my mechanic, the engine probably is shot
I'm sure a revscene member will buy the car off you for $1500
but to be semi serious with you,
I'd check the fuel filter --- I imagine it's the original after 259,000 kms on cheap canadian gas, it may need to be replaced (I had similar problems on my jeep at 130,000 kms, replaced fuel filter and MPG went up + no shuddering under acceleration)