A rowing machine would definitely work for cardio but personally I could never get into it, and have put a lot more hours in on a treadmill and pavement/sidewalk pounding. He's got a point though it is high impact and potentially you're setting yourself up for problems.
Step machines are a great burn and take up less space in a home gym, and if you use them right (people tend to lend more and the handlebars rather than keeping a straight back) with fartlek training then you can get a serious burn-on and build up some serious thigh muscles that'll have you bounding up hills if you later decide to expand to outside running. Very low impact and digital ones will have an incremental warm up programme - and they're dead easy to watch tv, adjust your music, whatever.
Personally I'd go for a cross-trainer/ellipitical trainer if money isn't an issue and you can get a decentish model that designed for something more than casual training. They're a cross between the step machine and the running machine without any of the impact. I've found them great for core muscles and tightening abdominal obliques (tightening the flanks is actually the secret to flattening the stomach once you've cut most of the fat) plus you get that upper body workout at the same time. Only thing with a cross-trainer is some people just can't get with the motion which can be a little odd, so try first, but in my opinion they're the best all round trainer and most enjoyable.
If not go with the stepper and buy yourself some light hand weights to pump your fists too as you step. When I say light, I mean light because if you pump the air with those things for 20-40 minutes at a time on a step machine your neck and shoulders are going to be popping out when you stop.
If you're not used to exercise don't bother with a pull up bar yet. Start with some press-up variations and tricep dips. Test your resolve first before you damage your doomframe. If you've already got space for treadmill you can look into a chin-dip-leg raise stand alone piece of equipment later which you should be able to pick up quite cheap.