I built two dynamic divers 30-40 years ago and had a lot of fun running them, before I moved up to static divers. They were wood hulls, and one is stll in good working condition. They need considerable forward speed to submerge, and cannot lurk on the bottom, because they are positively buoyant. That is a safety feature. My suggestion would be to buy the kit and build it as per plans as a dynamic diver, and convert it to static diving, with a ballast system, after seeing the problems of sub construction, trimming, and driving. It's nice just to get something in the water, and then to save and dream and plan and do research for the next stage.
Your air frequency radio is not legal for ground or water use, even though it might work. Switching crystals to make it legal at 75Mhz might not work, because the radio would probably have to be retuned professionally to restore it to adequate range. Radio South could probably convert it, but it might not pay. Research cost vs. new or used surface frequency radios. Hope this info helps.
RCABS- It's easy to control your own bladder!