Money: You will need to plan for c£30 for a tips kitty + your own choice of tip for the main leaders at the end. You will not need money for lunches at Yala as you eat a massive late breakfast after returning from morning drives. Currency rates were competitive at hotels as well as airport. I only spent £120 including all tips.
Camera etc : 400mm quite enough for the whales, which are quite big! You may want a longer zoom for the birds in Yala. I found video clips via the camera were the best way of capturing the whales.
Hotels: Yala is superb, completely faultless. Mirissa the hotel was in itself fine and fronted onto a lovely beach, but could be better managed. The lock on my room patio was a bit dodgy in the 2nd stay, and food on two nights was poor. The whale watching trips seemed to be based at the hotel-as the whale trips were really well done it might be tied in with this hotel being used? Nothing wrong with it that some more attentive management couldn't improve.
Yala leopards : I had one sighting at close range but at the dark side of dusk, another clear but at binocular distance, and another of leopard and cub at long zoom through thicket. I don't think you should make leopards the be all and end all. Yala is very rich in birds (eagles, owls, bee eaters etc etc), mammals (including elephants) and reptiles. If you just hare around from place to place trying to find a leopard you will miss a lot.
Visa eta: They now seem to have sorted this out.