IR is necessary, otherwise you would only be able to fly in good weather conditions, and you wouldn't need to buy a 155 anyway. The salary for a corporate pilot with ATPL, would at least be £70k, and don't forget pension, company car, company phone, laptop, health insurance, medical cost, training, proficiency checks. You might be lucky to find one in these (bad) times.
You're talking about buying an EC155 for millions, why don't you want to pay for the most important 'part'....