I have been spearing for 7-8yrs and no other form of fishing compares.
My advice is, if you are seriously keen to get involved, buy the best gear you can afford.
starting with a high quality silicon mask, $80-$140 just for the mask is a good range, i have found black silicon to work better than clear.
Next is the gun. Me and 2 mates started at the same time. They both bought sea hornet tourists, I got myself a omer excalibur 2000. It was just over twice the price but i still have it. Same clips, same shaft, only replaced the rubber. Theirs lasted all of 6 months.
I also have a rob allen that i cant fault.
A sharp knife is also important. My first knife, a very cheap land and sea had more chance of infecting the fish with hepatits than subduing them quickly. a quality knife with a quality SS blade is a must. I have been caught up in my tow line before, luckily enough the fish wasnt big enough to pull me under but this does happen.
Like you already mentioned, a float/flag is a good option. Bigger and brighter the better. A mate just used a white boat fender, next thing a skipper not paying too much attention came past and picked up his float and gun while diving off SWR and his tow line was only 10m long. The bigger and brighter your float is the easier it is for others to see. If you intend on diving blue water, very very important one a bit of chop starts.
Lastly, if possible, most important is a dive buddy. too many stories of shallow water blackouts (swb)
a good accessorie but not nessessary is a decent wetsuit. I use a beachet suit that cost me almost $500. It is a pain to put on but the flexibility and warmth is awesome.
Unsure how a kayak would go used as a float. would be fine in calm conditions. would be no good in significant current, swell and near the rocks..
thats covered a few points which i think are most important with diving