Buy what you like best but please don't buy a nice original car and then cut it up. Bolt on's (if you retain the original parts) won't hurt or help the value significantly in my opinion. The cost of mods are seldom returned at resale unless the labor is free (IE DIY).
As for value, search all the typical sites to see what they are selling for and go from there. Watch dates sold when searching because prices seem to be rising and a 2 year old sale will be below current market.