Good starter gun? Yes. For the Uk and places with limited visibility. The waters of Florida can be gin clear - in which case a longer speargun might be more appropriate (100cm+)*. But as long as the fish come within range you should be ok.
Does it float? I would be surprised if it doesn't. Most spearguns are intended to float. Some spearguns only float without the spear (e.g. after a shot has been taken).
Does the spear-line float? It doesn't matter but probably not. You float-line (the line to your diver-down marker float/buoy) should float though. Nylon rope doesn't usually float but (usually much cheaper) polypropylene ropes usually do float.
Is it hard to set-up? No, pretty straightforward: take a look at the excellent
Dummies Guide to Rigging** a Speargun thread on this forum. This short thread is particulary apt:
https://forums.deeperblue.com/threads/cressi-comanche-mono-line-crimping-botch.102217/#post-938713
and/or youtube.
Is it good? Yes, can be very good value (depending on the price you pay of course). It was used, in various sizes, by a spearfishing World Champion.
Reel? Yes, you can use a reel with it.
Powerhead?! That is a very specialized piece of equipment and I don't feel qualified to comment on it beyond saying that they are generally used on much larger, heavier equipment and for dealing with, I think , large shark - which most spearos avoid (I recommend that you avoid sharks - for environmental as well as safety reasons). You don't need powerheads for the vast majority of spearfishing, even for very large pelajic fish*.
*Note: Very large fish do however require special consideration, equipment and tactics. Not really a beginner subject for most.
**
https://forums.deeperblue.com/threads/dummies-guide-to-rigging-a-speargun.64504/