If you can do that anticipated volume -- get a real merchant account with a direct processor first.
Then add some third party processors and alternative payment methods in a cascade to capture the remaining conversion possibilities.
We do well with Epoch's PayPal processing as an alternative.
If you can offer to your customers the option to buy for 12 months at a special price, and pay you direct by ACH or SEPA direct from their bank, bank-to-bank, after one month's purchase -- you will save a lot in fees
