First off I think this is a perfectly sensible question. I don't have much experience with FMA but just some thoughts I have:
i) You are spot about Silat. I've seen some great conditioning in Silat classes.
ii) Any sort of movement exercise is going to help someone who isn't exercising regularly, be it FMA or something else.
iii) I've done a little fencing before, and it is a decent workout. Some FMA martial arts seem to have rattan fencing, which I expect is a decent workout.
iv) I understand if someone is short on time they might want a one stop shop for all their exercise. Certainly martial arts like Karate or BJJ can provide a pretty good holistic workout. That said, there are a lot of other ways to keep in shape. I've a friend who runs marathons and he did Aikido for a number of years. He actually prefers low intensity martial arts as his body takes enough of a pounding with all the running he does, plus his running keeps him in decent shape so he doesn't need to worry too much about whether or not a martial arts class with get him fit (aerobically speaking).
In short, do FMA and see what its like. Some martial arts classes, even in they teach the same style, will be more physically demanding than others. If you like FMA, keep doing them. If the class isn't a great workout, you can always do other exercise to help keep you in shape. Certainly from what I know of FMA something like fencing or boxing might be a good supplementary system to study alongside FMA.
"Let your food be your medicine, and your medicine be your food"