The lat pulldown would be a great combination to the things you're already doing. I highly recommend avoiding the leg extension machine, especially if you have knees that aren't perfect - it can put a lot of stress on your knees compared to other ways to use those muscles. As everyone else has said, generally free weights are better than machines, but the lat pulldown/row machine is a good option.

You could also think about incorporating some bodyweight exercises into your routine. Squats, planks, push-ups, lunges are all really great at strengthening your legs and core and you don't need anything besides a floor and maybe a wall. The nerdfitness site Rydra linked to in the last post has whole videos of ideas for these, and the crossfit.com site also has videos - so you can work on making sure you have proper form.