Yes the seals can be replaced without splitting the motor.
No, a carb kit isn't that hard to put it. Rebuilding a carb, that is different. $300 for a real rebuild, install and tune, well..... that's a little high, but not really all that far off. $400 to put seals in... that's high. Even at the rate I charge.
Go to the tech section and download the manual on the M18. There are only like 100 carb rebuild threads and seal replacement threads on here. Just do a search.
