I also have a studio in my house with about 1500 sf of space. When I purchased larger pieces of equipment, I was always looking for multi-functionality. Here is a list of the pieces I own and would buy again if confronted with the same question:
Bowflex – This machine can easily be configured to resemble health club equipment. A great plus is that the resistance starts very, very light, making it very suitable for beginners, the deconditioned and rehab scenarios.
Total Gym Power Tower – This is my personal favorite. What I like the best is that you can do a lot of things on it that are impossible to do in a health club. It has a lot of accessories and can even be brought to the function of a Pilates Reformer.
Paramount Dual Axis Cable Tower – This is one of those cable machines with moving arms to the side. I like that each side has its own weight stack and that the arms move both horizontally and vertically making it extremely flexible. I also use this piece of equipment as a place for balancing exercises. I lower the arms to create a cage and thereby a very safe scenario for balance exercises, standing on a Bosu for example. This really builds confidence in people.
On the cardio side, I have a treadmill, an elliptical trainer and a recumbent bicycle. If I had to pick one, I would go with the elliptical trainer, then the treadmill. My bicycle is used rarely, and I would not buy it again.
When purchasing machines for your fitness establishment you want to consider multiple uses by multiple types of persons. I think that Free Motion has done it best in the versatility department by concentrating on functional motion. When it comes to cardio machines, you should step on them and try them out first to make sure its a good fit for YOUR clientele. I have a Cardiozone treadmill that isn’t fancy but it has a 22″ wide 62″ long belt and a 500 lbs weight capacity; I happen to train very obese individuals so its a great fit… for ME.
I love Life Fitness when I use machines. If I had just one machine to work with it would have to be their Dual Adjustable Pulley with a vest and multiple handles. Very versatile. Takes up very little floor space. It’s a workhorse!
That being said I do very little training with machines and use mostly dumbbells, barbells, body weight, stability balls, TRX, BOSU, etc