I would do the plugs, but they are probably not the issue. A hesitation like you describe can often be caused by the fuel mixture being off. I forget if too lean or too rich causes it. I had a car I was tuning, it was so bad I could stall it if I went WOT with the peddle too quick.

If you think the O2 is due, it would be a great place to start. You can usually get one for about $30 now days. A universal type.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car