I must say that in part I have to agree with a contractor saying that they "may" have to replace the gutters, many times depending on the age of the gutters and the attachment method (esp. with pins driven through the lip into the facia) removing the gutters causes damage in the way of tears and dents in the guttering. Some contractors will automatically price in new gutters just so they don't have to fuss with the old ones.
Rotted facia is usually a good indication that your gutters are not working correctly and or there needs to be drip edge added to the roof.
$5 - $7 a foot for installed guttering seems a little high but that is always going to be dictated by location.
A good contractor should be able to replace 12' of facia without too much trouble and time.
If your still unhappy with the quotes you have, the answer is as simple the keypad on the phone. Remember your the client, the contractor works "for" you, if you only want 12' of facia replaced then tell them that's what you want and nothing more. (just make sure that 12' is all you need replaced first

)