1. Croatia uses a 230V/50Hz electrical supply, which is the same as in the UK. Type C & F sockets are both used there:
http://www.worldstandards.eu/electricity/plugs-and-sockets/
A simple travel Poundland adapter is all you'll need:
http://www.poundland.co.uk/electrek-travel-adaptor-2-pack
2. Croatia uses the Kuna as its currency. Larger foreign exchange places in the UK will have it in stock but you can also get it from any Post Office branch by ordering it online:
http://www.postoffice.co.uk/foreign-currency
You can bring Croatian currency out of the country and change it back here.
However remember that it's just as easy to use a normal UK debit card at Croatian cash machines (or to pay for goods and services), so you won't need to take much cash.
3. Croatia is in the EU. All EU citizens have the automatic right to travel throughout the whole of the EU, so (assuming that you're British, or another EU citizen) you most definitely don't need a visa.
4. Yes, they drive on the right,
5. Croatia uses Central European Time, so there's a one hour time difference.
There's loads more information on Wikitravel:
http://wikitravel.org/en/Croatia