Do not worry about safety at all. Compared to many African countries it felt very safe.
Best currency to take is US Dollars, which are easy to change. Theres no time difference and UK plug sockets.
You don't really need much money at all. Apart from tips and the odd drink, the only things are any optional activities at the end for time in Freetown, e.g. a trip to Bunce Island or city tour.
There are a couple of long drives going to Tiwai and back, but the roads are good for most of it. Camping equipment used was of a high standard. Don't expect too much tourist infrastructure, but hotels were simple and clean.
The country has all the elements (beaches, wildlife, scenery, history) to be a great destination, but tourism is still just beginning, so service etc. may be slower than you are used to. That said, if you are considering a trip to Sierra Leone, its likely you are a seasoned traveller already! We didn't stick exactly to the itinerary, so a flexible attitude is needed, but thats Africa.