If you could have only one exercise machine, I think a treadmill would be the best. This is because it can run faster than you can, making you to work harder and as a result you will lose some pounds effectively. You also need to visit a fitness specialist on advice you on which way to lose the excess weight.
At 73, overweight, I'd be careful not to over-exert yourself, regardless. Or, without a doctor's OK, first. Suggest low impact equipment like an exercise bike. Less stress on your hip & knee joints & will still get your heart & muscles moving. Like treadmills too.