I'm not sure if a cock ring is advisable for people with diabetes, as diabetics can get circulation problems and a cock ring cuts off circulation.
Viagra is quick acting and lasts for up to 4-5 hours. To feel the effect your penis must be stimulated.
Cialis is slightly slower in acting, and you don't get so much flushing as with Viagra. The effects last up to 36 hours. Some people get a headache from it, and others get muscular stiffness. Here again you need to stimulate the penis to feel an effect.
Some people swear by papaverine. It must be injected directly into the penis shortly before sex.
Other solutions include an inflatable implant.
The best solution for you to my mind would be to lose weight and regular intensive exercise. Your equipment will start to function better then. If you have low testosterone your GP might suggest using a testosterone gel to raise testo levels. However she/he will almost certainly insist that you improve your lifestyle as well.