I think if there's one thing you have to take into consideration, then it's the amount of content that you will have to learn.
A lot of the time the topics aren't too hard to follow, providing you give yourself sufficient time to process and understand it all.
If you do plan on taking Further Maths, I would highly recommend you pursue revising and studying from very early on.
The Maths itself isn't always necessarily a brain ache, it's just people make it harder for themselves because there is so much content to learn and they don't seem to realise or come to terms with this until later on, and it is of course harder than the A-level maths, but not impossible.
If you leave it fairly late, then having to learn/revise for so much content in such a short time is what can really affect your grades and success. This is certainly what has affected my friends. They say it's not necessarily too hard, it's just there's a lot to do and they haven't given themselves that much time to commit themselves to it, so that is primarily what makes it hard.
As I said, set foot on the revision and studying from early on to allow yourself the time to prepare yourself and retain everything, and then it shouldn't be too much of a problem.
There's this whole "1 in 1 out" phrase that a lot of schools use.
For every 1 hour of lessons you have in school for a subject, you should be dedicating 1 hour of study outside for that subject. This should be fine for FMaths, but considering there is a fair amount, a little more may be required.
Considering you did iGCSE and FMaths, then you already have a slightly better grasp and understanding of it all than others, but it's best not to become too complacent as this could be your downfall.
I don't know you, but from the little you have said, I think it's suffice to say you have more than a reasonable chance of doing well if you commit yourself to it.
Providing you dedicate enough time to your other subjects, then it shouldn't cause a problem or shouldn't interfere with your success in other subjects.
As others have said, you can always browse a few of the topics and see how you cope with them.
If you're doing it in school, you always have the option of attending some lessons and seeing how you deal with that, so that you can make more of an informed decision as to whether you would like to continue with it.
This is what I did.