I would go along with the comment above about it being cheaper to change the car. Whilst I've not been able to find the information for an IS250, Toyota do have a technical document showing the installation on a Camry, and it involves all sorts of things, removing the centre console, top of the dashboard, all the trim down the side of the drivers seat, including the seat itself, lifting the carpet, which in turn means removing the footrest. I would say thats why usually these things are stated as factory fitted options because there a lot easier to put in whilst the car is being assembled. Built in Sat Nav systems usually run at around £2000 so if you add that onto the cost of probably 5+ hours of labour and the risk of everything not going back together quite as it was originally I would say try something else.