Oh I'm not all knowledgeable you know I just seem to know small amounts about lots of things. May be due to Asperger's Syndrome... possibly.
I left retail in 2003, so things may have changed a bit. AVS is now pretty standard for CNP transactions, whereas in pre-2003 days, I used to have to phone the card processor almost every time, to get authorisation as the machine would not do it without a phone call - AHH! Transactions were high value though, often £3,000 plus.
Ask around a bit, see which companies will even consider giving you a quote. Before shopping around, try to estimate how many transactions per month you would be likely to be putting through, and the amounts associated with that.
Check out the trade associations offer, as that will have been designed specifically for B&B owners needs, it may even give you better rates - and is likely to give you a lower initial startup cost, as it can often be the setup fee and terminal rental costs that get discounted.
Terminals used to require their own phone line, not sure if that is still the case or not - but certainly you won't be wanting it on the same line as your incoming bookings calls. So you may need to factor in the cost of a new phone line. Contact terminal providers to confirm what their terminals would work with.
Some systems like Barclay's ePDQ-Lite work via the internet, but I am not sure if that gives you a terminal, so not sure it it could be used for card present transations.
As a B&B, I expect you want to:
Accept deposit payments for bookings via Phone/E-mail/Web (Card Not Present with Address Verification)
Accept payments directly from the guest - such as settling accommodation bill, bar bill etc. (Card Present, with PIN verification)
Something else to look at is what cards to accept. I found that accepting Switch (now Maestro), Visa Credit, Mastercard Credit, were sufficient. Though I did have customers who wanted to pay via Ammerican Express. If you get quite a few international guests, you may want to look at accepting American Express (AMEX) but the transaction costs can (at least they were 8 years ago) be a lot higher.
Perhaps survey your guests, by having a question on their checkout survey asking what card types they would have liked to pay with.