Some food for thought: Free advice, probably worth what you paid for it.
IF you want to mod an early 911? Be one of those "R Gruppe type" cars? You'd better choose your mods carefully. Even then, you will limited your customer base if you decide to sell the car.
As illustrated here...some may not like your ideas on mods, they may even find your lovingly done modifications to be distasteful. No matter how much money was spent.
But hey, we're not into playing with these cars for the money, right?