Are merchant accounts necessary? Can some other bank services be used when charging clients while still being professional?
I'm thinking of using wire transfers or maybe PayPal. Can PayPal or Google services handle 6 figure transfers and payments?
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.