They are not necessary, and you can still look professional using other service providers.
You can use PayPal Pro to do credit card transactions but I have a feeling they would have concerns of doing a 6 figure transaction. Even if it did go through it's possible PayPal would hold on to it for a long time before releasing it to you incase their was a charge back / fraud.
You can also look into www.stripeapp.com which doesn't require a Merchant account to collect credit cards (just a valid US bank account).
If you do go the merchant account and processing gateway route you can look at:
http://www.braintreepayments.com/ They are pretty popular now and can help through each aspect. They also have plenty of info on their site on how the whole thing works and what is a merchant account, gateway etc.
Typically 6 figure transactions would be check or wire transfer
I'm not sure where you are from, but you could also look into using Dwolla.com It only costs $0.25 per transaction over $10, and under $10 is free.