Whether you need longer shafts is also dependent on your arm length not just your height. I'm 186cm too and play with standard shafts but you may need to have slightly longer shafts. Shaft stiffness depends on your swing, swings with faster speed allow stiffer shafts so that depends on you too.
I would recommend you go out and do some research on what you want: blade or cavity-back, graphite or steel, regular or stiff shafts, premium or value brand. Do this by testing at your local range and reading a few golf magazines and then decide how much you want to spend and what exactly you want.
I bought my
irons (Mizuno MX-23) brand new from ebay and have been very happy with both the price and the performance. It also helps that there is info on the seller (via feedback), a compaint reconciliation process and that you are covered to a certain extent by PayPal.