I agree with @jaagare; on many things although at the end of the day, it is up to you @blee088 ;
Any business has its risks. You can loose everything even with a more traditional business. And that's why you should investigate carefully and decide if you want to do.
Do not only investigate the figures (you could try alexa and google and other tools to know if what he says is true. Also try to find his ads and investigate how that income is coming from), also investigate the seller and his reputation.
Maintaining a site is hard work, and needs dedication and time and lots of nurturing and energy. And very specially if you don't have experience in the field, and not even knowledge.
Let's say that his figures are right. The income will drop in the very same you take that domain (how much? who knows?)
Even if the seller guides you through the steps to retain that income and maybe even making it grow, there will be a moment that you will be on your own.
The only way that you may have a chance to succeed is if you're ready to work hard, study hard and move with times. As with any business, you'll have to update strategies, change them completely, see what works, see what doesn't, add new stuff, etc.
It is going to be tough ride, and there's a learning curve involved. Not as high as if you're becoming an engineer but it's not as easy as many people say. You'll be experiencing first hand the amount of rubbish that some "gurus" speak about SEO and internet marketing... to find that it is not true.
It is going to be stressing (even more if you have another job) and you simply need to love it to keep on going, and you need to measure your energies so you don't burn out in the first months and through everything away. It is a matter of making the right decisions, learning from mistakes and being constant.
Summary: If you don't love backlinking, don't even think about this web. If you do and are willing to spend the necessary energy to make this site grow, then do your investigation in relation to figures and the seller's reputation. Make a calculated risk, don't jump just becuase it looks profitable.
Edit: if this business is not for you, there are lots of things that you can try and get that income you want