Welcome. How old is your Futaba? Do you know if the frequency is still good for surface/boat usage? The old style (G style) servos had pins (male) that would go into the receiver (female). Current style (J Style) is the opposite it is pins on the receiver. Other brands of modern servos are the J style. You can see a picture here: http://forum.sub-driver.com/showthread.php?1652-Newbie-no-more-Building-the-Revell-1-72-Gato/page11&highlight=futaba
. There are cables out there that will convert the old style to the new one..... Tell us about the Futaba you are going to use, what model, frequency?
Curious, What are you wanting to build? The use of two speed controllers is not unheard of, just not common or not necessary.
Are you going to use static or dynamic diving system?
There are easy subs to build and there are difficult subs to build. Price can be all over the place. Depending on your skills, starting to build a sub from scratch can be very frustrating, but we will help as best we can. For your first sub, my suggestion is go simple and just get it to work. That alone will help you from getting caught up in so many different ways you can build a sub. I see many start, but few finish because there is a lot to learn and challenges to overcome. If you start chasing all the different do-dads that you can put on your sub, you increase the chances of stalling and putting it on a shelf to collect dust.
Tell us more about what you are going to do, the more you provide, the better the guidance we can offer.
Oh, and post your build here.
If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.