I'm assuming that you want this stuff for snorkelling, rather than swim-training or SCUBA diving? Regardless of intentions, fins should be the right size for your feet, especially if you're buying full-foot fins. If they're too tight they'll give you cramps (because circulation will be impaired, or your foot will be 'curled-up'), if too loose they'll chafe and/or may fall off during use.
Use your actual shoe size as a starting point, but do actually try the fins on, because different manufacturers may use different 'standard' foot shapes for the footpockets, or their size ranges may be different (e.g. one may produce 42-43/44-45 fins, and another 41-42/43-44). Shop around, and buy the ones that feel most comfortable. If buying online, check the retailer's refund/replacement policy
(All the reviews I've read say that) yes, less effort is required to swim with split fins than single-blade fins, since your kicks can be somewhat shorter, increasing your streamlining (this is especially useful when surface swimming). Tusa, Atomic and Scubapro all do full-foot split fins--although strangely Apollo, the originator of the 'Nature's Wing' split-fin concept, apparently does not.
As for 'fins for your hands', there are two options, but neither are really useful for snorkelling. Hand paddles strap to the palm side of the hand, so you can't use your hands for anything else while you're wearing them. They're excellent for upper-body swim training, but useless for diving (and I would argue, also snorkelling). The other option is webbed gloves, but these are really only useful for snorkellers/divers who've lost the use of their legs.
If you can, it's better to use just your fins for propulsion, since leg muscles are so much more powerful than arms. (Also, it keeps your hands free for e.g. gear adjustment, or a camera.) But it's your money, so you can do as you please with it! ;-)
Any further questions, please feel free to email me through Y!A.