If you are visiting Istanbul just for business, stay wherever is close to your business, or you may lose hours in heavy traffic. Otherwise stay in Beyoglu district, and as close to Taksim Square (said to be the heart of entertainment) as possible. There are many hotels suitable for your budget. 5 star hotels with Bosphorus view mainly lies in Maçka and Harbiye region, and there is one just in Taksim Square. There is also one in “Ciragan (ÇıraÄŸan)“, an old palace by Bosporus. There are also many touristical hotels for much reasonable prices in Historic Peninsula, but personally I am not a fan of the neighborhood, hence I’d prefer Beyoglu district. In day time, you can take a cab (appr. $10) or alternative transport and one or two days would be enough for Historic Peninsula. I will be giving details about the Historic Peninsula and Taksim Square later.