Turnover is irrelevant, you need to be looking at the companies profits. Don't forget there's staff, rent, stock, plus a whole lot more to take away from that figure. Is the £8k a week an average? Because I'm certain its not going to hit that amount week in, week out. Take a look at the companies finances and maybe have a professional look over them too if your uncertain. How long is it before your going to see a return and is it really worth it?
You've also got to think whether your going to be able to run your current business as well as the newsagents/ commodities store? If not, what effect is this going to have on your income/ lifestyle?
Just a few things to think about...