There's a guy selling completely rotten GS500s in G10 or I think G11 Markaz for 180,000 Rs. Which is absurd, but if you can bring him down to 170K, pick it. From there on, remember you have to spend another 100,000 on it to make it road-worthy. This means new Tyres, discs, caliper-repair and pads ( around 30K for these only ).
Then body work, including rebuilding the engine as those old MP and Highway Police bikes are like 80,000 kms on em. There's a guy who restores these bikes in Gawalmandi , I forgot his name but his shop is right at the corner of the left turn you make instead of going straight under the bridge on Kashmir Road. Turn left and the first shop at the first left is his. He charges around 40K for labor but does a really good job including replacing clutch, rebuilding the engine and replacing pistons and camshafts etc.
If you want something 500cc , opt for Suzuki GS500 as it runs on Carburetor and Carbs are easy to work with than EFI.
For Carburetor, especially Buttlerfly carbs, I'd recommend Malik Iftekhar Ahmed of Bismillah Automobiles at Marir Hassan ( near Shaheen Chemist ). Tell him Aqeeq told you about him.
