if you restrict your payment to cryptocurrency only then you restrict your customers because not that many people even know about cryptocurrencies and those who do know are using them as investment not currency specially altcoins!
what you said about giving discounts for crypto-payments is a better idea as long as you keep the fiat as the normal way of payment.
also be careful with altcoin payments, they are not as safe as they look. and many things such as double spending is easier using many of them.
Btw, why not use third-party payment processor? Should've solved a lot of issues such as conversion rates and so on.
there is no point in accepting cryptocurrency if you are going to use a third party processor and/or receive fiat from them.