Getting back in shape... and I need your advice

I'm 21 year old male, 5'9, 150 lbs.

It's been about six years since I've been active and I want to get back in to shape and build some muscle. I started working out at the gym about a month ago and have fallen victim to using the weight machines that I hear being warned against in the fitness FAQ.

I've read through the FAQ a couple times now and the info there is great, but I'm a little discouraged. Everything that I've read says not to use weight machines. But there seems to be some mixed opinion when it comes to pulley weights so I'll post what I'm working with to give some context. One of them looks like a lat. pull-down except it has two ropes with handles instead of just a bar. The second one I use looks like this. And the third one looks a little bit like this except, once again, it uses ropes with handles instead of bars.

My first question is are these machines completely useless? I've been using them about 3-4 times a week for a month and I can already see and feel an improvement in my strength.

Also, I've always hated the standard sit up and usually get too much lower back pain to do enough of them to feel and burn. Since I found this machine I feel like I have more control over my situps and can feel a burn much easier. Is it worth it in the long run? Should I ditch this machine and stick to alternate methods? Which exercises work best for people with lower back pain?

To let you all know I have spent quite a bit of time reading over the FAQ and I would love to try out the stronglifts 5x5, but I don't really know where to start. I spent today working on my squat form and I can barely even do one without falling over (and that's with a broomstick). I'm honestly afraid that if I tried to do one at the gym with actual weights I could hurt myself or someone else. Same thing with bench pressing. I've never tried it and have nothing to practice on before I go to the gym.

I have a friend who could spot me, but he is just as inexperienced. He wouldn't be able to help with form, and from what I've read it seems like form is VERY important with bench pressing and squats.

The second option I've looked at is the simple fit exercises. Except there is no room for a chinning bar in my place. I live in a small apartment and every doorway is lined up with the wall, leaving no space to lock the bar in.

And where to you guys stand on leg presses? Are those out of the question as well? I know this is a lot of stuff to ask about, but I would greatly appreciate any advice you have for me.

tldr: Are pulley weights okay to use? What about ab machines and leg presses? I'm thinking of starting stronglifts 5x5; any tips for beginners? What about simple-fit? Is a chinning bar absolutely necessary?