scaflock

At a span of 50 inches, such a design would make for a nice slope plane but not all that great a thermal flier. Ailerons are nice on a slope plane but like BMathews said, they can get you into trouble if you're not used to them.

At 50 inches, to be a good thermal plane you would need to keep it as light as possible. Think something like a DLG here. Light, strong and with a good airfoil. The drawback to that layout is that they're not really good in winds. If you ballast it up, you'll increase how well it can penetrate the wind, but loose it's floater characteristics.

For thermal flying, the old saying of "bigger is better" is very true. At 50 inches, your plane would fall well below the 2 meter span range which is by many considered to be the lower end of a good thermal ship. I for one love the 2 meter class and have had some great ships in that range over the years. If built right, they will handle slope flying well and still be able to thermal. If you have room to build and haul a 2 meter ship, I think that may be more towards what you'd really like in a "Combo" ship.

There's also the airfoil to consider when designing a sailplane. This is usually chosen by the type of flying that you intend to do with the ship. Most thermal ships have airfoils that are flat to semi-symmetrical in shape while many of the fully aerobatic slope planes will use an airfoil that is closer to being fully symmetrical. The projected weight of the ship will also have an impact on airfoil selection.

As to sheeting the LE of the wing, It has both good and bad points to it. The good being that you get a much cleaner shape to your airfoil LE as well as extra strength in the wing. The bad being that you end up with a lot more weight to have to provide lift to over come. My current design project includes a full "D-tube" leading edge and full house controls (Flaps, Ailerons and Spoilers) on the wing but I've also got 196 inches of span to provide lift. AT 50 inches you're going to need to keep it as light as possible or you may end up with a plane that will only fly on a slope with strong winds hitting it.

EDIT:ANOTHERTHOUGHTHERE...

At 50" X 30" you're going to need a lot more lead in the nose to bring her into balance. The further back from the wing you have the tail feathers, the more weight it's going to take in front of the wing to offset that weight. Look through some of the on-line shops to get a better idea of proportions of sailplanes that are already flying and on the market. The "wish-books" can provide you with a lot of information. Span, length, airfoil used weights etc.