I definitely recommend the 2nd dog. As said above, I am going thru this same thought process with Daisy. I don't want to have to end up worrying about health issues, but even then, Wesley can have them too. Like what was also said above, even the best bred Cav's have issues. With what we paid for Wesley from a really good breeder, I knew I wanted insurance to protect my investment, etc. My husband is NOT one to shell out money to keep a dog alive. Though if it comes to it, I'll find whatever way possible. And with Daisy, she's from a BYB but is in great health still at 2+ years old. Where as Wesley, scratches, has dry skin, barks, etc. Daisy doesn't bark a single bit. She is SUCH a blessing!
So Wesley is almost 5 months old and we got Daisy, who again is 2+ years. Wesley was ALL about it, but he's still just a playful little boy. Daisy on the other hand, wasn't fond of him at all to start with. But we've had her just a few days now and she's wanting to tease him and chase him. She finally kissed me the 2nd day at our house and is realizing that I'm her new keeper. She's REALLY starting to shine. Wesley is doing great with it. You can tell he misses some of my attention though. Daisy is SUCH an attention hog. She will literally lay her head on my face! But they both really compliment eachother.
I must say though, going from one dog to two is DEFINITE chaos! At least in these first few days. I hope it settles down! The crazy parts are Wesley barking his head off at her (she won't bark at all), them chasing eachother in circles around and over the couch, and trying to take two potty out on the leashes. They keep crossing! I have the problem of having no fenced in backyard, so I have to take them on leashes to potty. But I have to be out there because we have coyotes.
So anyway.... to your concerns, just have them do a physical if possible. I'm taking Daisy for a check-up tomorrow. I want to make sure everything is 100% with her right now, before I make the commitment. The family says she's in great health, and I do have proof she's up to date on shots and some for the next few years. But I want to make sure that there's not a condition they know about and aren't telling me. Which I highly doubt. They love her SO much, so they've been very thorough with me.
Anyway, good luck in your decision!
& two human rugrats!