How much venison you need is obviously dependent upon how many people you are cooking for. If it's only two or three, you may be better off with steaks but if it's for the entire family you should get a haunch. It's venison season this time of year, so you should be able to ask your local butcher to get hold of some, if they don't have it already. Cook the venison in the same way as you would a beef joint. For maximum flavour, cook it to medium-rare. You can cook it for longer if folks prefer that but after medium venison can get quite tough. Also, make sure that the fat and sinew has been removed (your butcher will do this for you if you need them to).
The chocolate sauce is easy. Make some beef stock (with an OXO cube for simplicity if you want), melt around about 300g of chocolate (milk or plain, whichever you prefer), pour in 175ml red wine and leave on a medium-high heat to reduce for a while. When you have a gravy-like consistency the sauce should be ready, taste it to make sure and add salt or pepper if need be.
Hope it works out. If you want a trial run, adjust the recipe accordingly and make it with a steak to see how you like the meat to be cooked. Enjoy!