My 93 only had 72,000 on it, GT vert. Had about 55,000 when i bought it in 01'. I hate cars with high mileage, much rather spend more on a low mile car.
I just use Castrol Super Clean ($2.99) at o'riley's, a brush, and dilute it in water. Spray it off real good, let it dry and then apply 303 protectant with a applicator pad. Looks brand new and my cars a 93 gt black top.
Try a clay bar. I used meg's Quick Clay you can get it at o'riley's or another parts store. It took out some black marks on my trim on my vibrant red 93 gt vert. I will NEVER polish and wax my car without using a clay bar first. I loved it.