Is the Fusion 600 good for serious leg training?

Yes for most home lifters. The hack squat and leg press run off a 310 lb stack, which is plenty for beginners and intermediates but short of what strong lifters load on a plate-loaded sled.

How much floor space does it need?

Plan for about 72 in of length, 52 in of width and 82.5 in of height. Leave extra clearance around the seat and press footplate for loading and getting on and off.

Does it include a leg curl and leg extension?

Yes. The machine supports leg curl, leg extension and upper-ab exercises in addition to the leg press and hack squat, using the same weight stack.

Can I add upper-body work to it?

It is modular and accepts Body-Solid accessories, but it is not a full multi-gym. Expect lower-body and core focus rather than a complete cable crossover setup.

What is the weight stack capacity?

The stack tops out at 310 lb. Resistance changes with a selector pin, so switching exercises is quick.

Is assembly difficult?

It is a large, heavy machine with many parts. Most buyers should plan on two people and several hours, and a professional install is worth considering.