re: First boat: Cal 2-27, Pearson 28 or Islander 28?
One thing to keep in mind is what else is wrong with the diesel in the Cal 27. If it does not run at all it is hard to know if it is just the head gasket or is there more damage. I know little about the farymann, but they are old engines and part availability might be an issue. If you go with the Cal you might want to negotiate an engine tear down to really get into what is wrong as part of the transaction. Another option would be to get the price down so you can plan on a total re-power if needed.
Pushing the Cal with an outboard is not a great option. It is 7000lbs so not only does the motor have to be up to the task, but the mount has to be very heavy duty and the stern re-inforced. It can get rough where you are sailing and having an outboard bouncing around at the end of the boat is not so great.