Bullet point advice.
*make sure your accountant comes with the highest possible recommendations because if they mess up and lose you money you can't sue them for negligence or malpractice, but they can sue you if you refuse to pay them because they messed up and lost you money.
*learn everything you can about the mystical 'stopaj'
*the Turkish equivalent to stamp duty is payable on virtually everything and is 25% of whatever you pay in rent or purchase price.
*When you ask any authority for imformation, never ever believe the first person you speak to, always check with at least 5 other people to ensure they all give you the same information. If there is any discrepancy between the information given ask at least another 10 people. The most frequently given, similar information is likely to be the correct answer, but don't bank on it!
*Be prepared to enact your own version of the Seven Tasks of Asterix whenever dealing with any authority.
*Be prepared to budget for lots of 'Don't ask, just pay' sums of money.
*Work out what you actually need to start your business and then double it and then take away the number you first thought of and multiply it by your mother's birthday.
*If your business involves bringing in goods or supplies from outside Turkey become an expert in the import laws here and try and figure out how much import tax you will pay on everything. Because it is very high in some cases. You can't just expect to have stuff sent over on a regular basis and avoid tax.
*Be prepared to pay tax on vitually everything you do (or think about doing)
*You need a Turkish partner, because foreigners cannot run a business in Turkey without one.
*Estate Agents here charge everybody involved in a transaction. They charge 3% (normally) and bewared the people who tell you how much they want then slap on the KDV (VAT) afterwards and say if you want a fatura (invoice) you must pay the KDV on top.
*Virtually every person you deal with when you first start will rip you off.
*Should you actually overcome all these (and a thousand more) hurdles and your business is even remotely successful, be prepared for at least 10 other people to think 'oh that's a good idea' and copy you.
*Don't employ a lawyer who lives and works in the town you are intending to do business in. He/she is almost certainly related to or went to school with anybody you are likely to find yourself in opposition with.
*If you rent a property, ensure you have a clause written into your contract (by your out of town lawyer)which fixes the price of the rent, because its not entirely unknown that in the unlikely event that after the first year of trading you are (or just appear to be) successful, your landlord will treble the rent forcing you to seek other premises.
*Be prepared to pay to have virtually every bit of 'official' paper translated by an official interpreter. (Expensive)
*Reasearch, research, research and then when you have finished researching, research some more!