If you're just looking to build a car to sell, the main goal should be investing money in features and attributes that are attractive to the broadest group of people possible.
Features like a well engineered suspension, and heavily modified interior or exterior may be worth a lot of money to the right people. However, the added cost of those elements will probably deter the vast majority of people who are just looking for a cool car.
So I think if you distill a pro-touring car down to it's essential pieces, you would end up with the following:
- RELIABILITY
- minitubs
- "nice" wheels and tires
- a decent set of disc brakes
- "finished" engine bay
- proper stance
- LS motor (I'd say for highest profit margin a stock LS2 with a mild cam)
- stock interior
- stock exterior