With regards to medication it really does depend on the type of heart condition diagnosed. If the Xray revealed a fluid build up around the heart then certainly a diuretic may help, but if she also has kidney problems (which can be secondary to heart disease) then diuretics have to be used with caution. Other meds which may help are ones that reduce the blood pressure, thus taking some of the workload from the heart and kidneys.
There is no one size fits all, so it will be best to wait to hear the specifics from your Vet. As to how long a Rabbit with heart disease can live, again no case will be the same. It will depend on response to medication, if there are other health problems etc. With my Rexes who had heart disease they had about a year of good quality life after starting treatment. But the condition does progress, sometimes rapidly, sometimes not.
Remember though, the Rabbit will not have any worries as long as they feel OK within themselves. Rabbits live in the moment, never stressing about what may happen tomorrow, next week or next month. My advice would be to take things one day at a time. Bella will hopefully respond well to treatment and be able to enjoy her remaining time with no worries at all.