One thing that you have to consider is that this isn't like buying a normal business. You aren't buying a retail business, where you can continue exactly as before, selling the same products. This brand is simply the name that an individual photographer traded as, you are a different photographer, the only thing carrying over is HIS name.
Clients hire a photographer because they like their work, their shooting style etc. As soon as you take over you will be somebody else. The brand name may remain but the product (your work) is going to be different. You will need to replace all the images on the website etc., with your own work, you can't advertise your services with somebody else's work - it's false advertising. You will need to pitch to the current clients again as a different photographer, the "brand" will be worth little as they will look on a different photographer as exactly that, somebody different and you will need to stand on your own work to get you bookings. With that in mind, it is not worth spending a huge amount on this business as you are really only getting a list of potential clients that you can try and get to do business with YOU.
You will be constantly having to tell potential clients who contact you, through the website or the social media, that you are not the person they think you are, you are a different photographer to the one who used to trade under that name. Do you have enough suitable images of your own to replace all the ones currently being used to advertise the brand? Do you have the skills to rehash the website and marketing to showcase your own work?
I haven't seen your work, but if it is not up to the standard these "high end" clients expect then using somebody else's brand name won't help you much. If it is up to that standard, then you should be able to get clients under your own name, without needing to use somebody else's name and reputation to attract them. If your work is not up to standard the brand you buy will quickly die as soon as you start disappointing clients.
You will need to make it very clear to clients, as well as on the website and social media, that you are a different photographer and the original photographer is no longer available. You cannot just take over and pretend to be another photographer.