There was recently an article in Excellence about this conversion. It was a DIY project, so that significantly reduced the cost of it. But you should still expect this conversion to cost you $5000-8000 depending on variables.
As for the value of the car? At this point in the game, 993's aren't something that have a lot of concern around originality. Only the lowest mileage or more unique variants qualify as "collectible" even though it's a car that has good value in general and has gone up a bit in recent years. These aren't '67 911S's.
However, at the end of the day, if you're concerned about value you should buy the nicest most original, lowest mileage car you can afford. Any time you modify a vehicle you risk compromising it's value. You need to decide if you want to buy a 911 and have it be "your" car or are you going to buy a 911 that you are concerned about keeping "perfect". It just depends on who you are and what you want. I am personally in the process of contemplating selling my 1968 911t because it's too original for an owner like me. I like to drive them hard and put them away wet. I like to modifiy and tweak them. My current steed is a poor match for someone with my urges.
