Umm... say what now?
Any details? That's a pretty broad, and significant, issue to be dealing with.
Did your doctor give you an ETA of when you can get back into training? Are there any restrictions the doc's told you to work around, avoid, or pay attention to?
Particular goal? Size, strength, fat loss, other?
In your case, though we don't know any more details, it would probably make sense to use one during your workouts (weight training and cardio).
Last time I checked, Polar is one of the top companies. I really don't remember the particulars about the different models, but they basically have more and more features depending on what you're looking for.
Unless you want to get uber-fancy (tracking calories, uploading to computer, etc.), I'd get the most basic model you can find from a reputable brand (again, I'd look for Polar). As long as it tracks your heart rate via a chest band, and maybe monitors how long you're training in your target zone, it should be fine.
I've known trainers who use on heart rate montiors with every client, trainers who absolutely hate them, and others in between. As I said, in your specific case, I think it would be a smart investment.