Regarding a kebab shop you will be up against a lot of already established businesses which could make things more difficult.
I'd be willing to bet that unless you got a prime place, you'll find it next to impossible to make it pay in the centre. Off the top of my head, you'll be running straight up against the place on Wielka that always has big queues, then there's Ali Baba on Gwarna and Roti on Sw. Marcin - not to mention the ones near Szkolna which always have big queues late at night.
There *is* a need for a decent kebab shop near Kaponiera - the one nearest to the night bus stop is overpriced rubbish. Apart from that, the market is saturated.