You can easily go to Thandiani on Chevrolet Joy... I have Chevrolet Exclusive 800cc, and I have visited Thandiani 2 times on same car.. Best route is motorway from Lahore to Hassan Abdal and then on GT Road from Hassan Abdal to Abbottabad than from Abbottabad to Thandiani....
Useful information:
Try to cover the area from Hassan Abdal to Abbottabad in very early morning in between (4-5am to 8-9am), on this route you will face so many traffic in day time Specially in Hari Pur...
About car, you need check your breaks, Clutch and Pressure plates....
For more information you can call me on 0333-8632388 (Imran)
Some Clicks from my last visit to Thandiani...
https://500px.com/photo/128583943/mountain-hut-by-captureaye
https://500px.com/photo/128505253/thandiani-abbottabad-by-captureaye
https://500px.com/photo/128505251/thandiani-abbottabad-by-captureaye