High blood pressure , more accurately called hypertension, is fairly common in senior cats. Most of the time it is secondary to either chronic kidney failure or hyperthyroidism. This means that there is not an underlying disease, as occurs in many humans. Because your cat's arteries are so small, a special instrument is required. The most common one used is based on the Doppler principle.
Hypertension is very treatable.
Cancer is another common disease in senior cats. There are so many forms of cancer that it is impossible to list specific clinical signs. The signs will be determined by the parts of the body that are affected. Therefore, weight loss, anemia, lethargy, loss of appetite, vomiting, diarrhea, difficulty breathing and coughing are all possible. However, it is unlikely that all of those would occur in any one cat.