[ben165]

It depends on the exchange provider. I use Kraken and they provide an unique address for every costumer. But I read some Exchanges require the memo field especially for Stellar addresses.

If you use a memo field and there is an error the order won't be found in the system. In this case, you can write a script which sends the funds back to the address - fees. In this case you don't lose money and you don't need humans to interact. A more difficult problem would be if sb. sends a wrong amount. Then you have to interact with the costumer.

Yes, you're right. Polling is bad but the easiest solution. The best one would be to listen to incoming transactions and take action if a new one comes in. I'm going to use this solution if I continue the project.