The origional MDS .40 had a poor carb on it that made the engine run poorly. You can send the engine (or maybe just the carb) back, and get a free replacement carb that will really make the enigine run well. That's where a lot of the "mixed reviews" come from, guys with the old carb hated the engine, but guys who have the new carb usually love it.

If the engine has been sitting for a while, gently turn it by hand a little. If you feel any "crunchy" feel, that's bad news, as it's likely rust in the bearings. If it feels smooth and loose (with maybe a tight spot at TDC, that's ok), then you're good to go. If it feels smooth but with resistance, that's usually congealed oil that will loosen up with some fuel.