One of the best things about an Elise is the completely changeable interior. If you don't like the door inserts and seats, you can sell them and buy what you want from Lotus or used. It seems like there is always someone here looking to buy seats and/or door inserts. So don't let the interior colour sway you one way or the other.
IMHO a demo car should not be worth as much as an owner car that has been loved and maintained carefully. You don't state what color the car is, outside or in. This actually does affect resale, and some colors are significantly more popular than others.
As for the supercharger, it's true, you can probably get more power for less money doing one yourself. Good luck, whatever you decide.
2013 Porsche Boxster (in silver and red, home at last!)
2011 BMW 128i cabrio, ///M Sport
2005 Elise in Racing Green with the touring pack (sold 8/6/12)
2007 MINI Cooper S in Sparkling Silver (sold)
Founder of the Racing Green Confederate