I have raced on a Catalina 27 for the past three summers. They are a real mixed bag. They offer a lot of room, and sail reasonably well for the money (should be able to buy a clean one should be less than $8-11K). In many areas like ours there are a one design fleet of the outboard versions and there is good close racing.
The down side is that these are pretty lightly built boats and they are up in years so when things get dicey stuff breaks. The big genoas and small winches take pretty strong crew members. The big genoas and spinackers are pretty short lived and are really quite expensive. On the flip side, the small mainsails last forever and are cheap to buy.
I like the Catalina 27 inboards for a starter coastal or lake boat. They sail ok and at least they don''t have a cored hull. Have it surveyed and don''t pay a lot. If you really like it then you will want a bigger boat so keep in mind that this is a interim boat for coastal sailing.
The words atomic 4 and diesel don''t go together. The makers of the famous atomic 4 gasoline engine, Universal Medalist Marine, alsomade diesel engines. My guess is that it''s the A-4 which is a very good engine. But like Jeff said the boats not new so get it checked out.
I really like masthead designs with big genoas like that C-27. Jeff does not. I think for cruising the big genny is easier for me. I would rather roll up a 450 sq ft genny that I can reef also than handle a 250 sq ft main. So the Genny is my primary sail. On a fractional rig you don''t get much of a jib. Whatever. They are both right answers.