This is once again a good reason to find a good vet. As noted, this form of heart failure can easily be mistaken for pneumonia, and if not diagnosed and treated properly and promptly, death will result.
The medications listed require a prescription, and cannot be obtained otherwise. Also, dosage amounts are not given here as dosage is based on a number of varying factors: weight, severity of congestion, etc.
It has been reported by people in different areas of the U.S. whose guinea pigs were diagnosed with the same condition that twice-daily use of the diuretic (lasix) may be preferable to once daily, and dosage amount may need to be adjusted to find optimal dosage. Consult your veterinarian before modifying any medication dosages.