Rowing Machines
Rowing is a fantastic exercise which involves most of your muscles and the rowing machine is also an affordable piece of equipment for your home gym. Prices range from £50 to £500 and depending on your needs / usage you'll be able to find a rower that suits you.
Magnetic resistance rowers are often the cheaper and these don't tend to feel as smooth during exercising.
Commercial gyms often use chain rowers such as the Concept II. These types of rowers without doubt the preferred choice.
What about back aches / problems
Rowing done correctly shouldn't affect your back and should in fact help strengthen it. Unfortunately most new trainers assume you use your back to pull all the weight / resistance from the rower. In fact you should ensure your back it perfectly straight push with your legs and then pull using your arms / shoulders and upper back muscles. Do not lean forwards and backwards whilst doing this exercise and try to keep your back straight.