Just got an '81 CHB 45 with twin Lehman 120's in Feb. I can only share that limited experience in responding to your questions.
1. What do I need to be cautious of in a CHB? What are their issues?
The biggest things for us is leaks and associated delamination, and fuel tanks in less than satisfactory condition. There is alot of work required for us to stop these leaks and repair the impact of those leaks. Other than that, I think we have the normal issues related to age. For us that includes some work to heads, electrical, batteries, teak finish, steering, shaft packing, fuel tanks and electronics.
2. Not to start the twins vs single discussion, but I had been leaning toward a single, but the economics of this situation are making me rethink my single position. Right now, the twins are the only negative on the boat. (of course have not had the survey yet) Any input on twins and the issues I will be dealing with.
Again, my personal experience is that the maintenance cost is double but the maneuverability in great and necessary in the tight confines of our covered slip. Of course, if you have a bow thruster, this isn't relevant.
3. What about Yanmar? Good, bad, ugly?????????
We have a Yanmar genset, which is great. But as mentioned, we have Lehmans for power.
4. Are there any major deal breakers I should be looking for?
I can't answer this well. For me, it would depend on your skill level across multiple disciplines, and/or your wallet.