I can only add a little bit here - I work almost entirely by hand, but in a large space with a heavy bench, it's filthy and I don't use the space efficiently. That said, if you need a relatively unstable work bench but want to use it for planing, you can put a lateral foot out of it far enough ahead of the bench that it will butt against the wall- at just below the height of the bench top. If your wall is questionable in strength, make the end of the foot relatively wide and wrap a towel around it or something.
that gives a light bench a physical stop. For quite some time, I worked with one of the budget benches that sjobergs sell. They are not that small if you get the larger ones, but they are definitely weak. I stock a rod out of the end of it if necessary and butted it up against a large machinists cabinet that someone left at my house when they moved out. It worked great.
(look away mike) the double iron is your friend here for planing things - if you learn to plane without having the wood tear, everything will be more controlled as you're dimensioning and you won't need to use many heavy planes. intermittent in and out of cuts with tearout will drive you bonkers with a light bench and maybe have you ramming through something. It's a bad way to work even with the heaviest of benches (you could beat up our surroundings, but with a heavier bench, your surroundings end up being up on you instead). controlled and in rhythm is what you want for all but ultimate hand work.