Medium,
Don't ever apologize for asking questions. It's about the best way to learn anything.

You will get excess fuel/oil blowing out the exhaust of every glow engine, it's normal. The residue is a pain to clean off the aircraft, and most people use some form of homemade cleaner and paper towels. (A mix of Windex with ammonia, and Fantastic or 409 works for me.) It should not harm covering or paints that are made for RC aircraft, most are fuel proof. (Some decals and graphics will lift at the edges though.) DO be sure to coat all exposed wood with epoxy though. The oil will soak into wood and soften it.

I'd say 'maybe' to the newer engine if you can afford it. K&B 40's (older ones at least) have a reputation for being finicky. The carbs get to be an issue over time too.

An OS 46 FX is one of the best engines out there right now, but also on the expensive side. Some swear by the Thunder Tiger .46 Pro (I think). It's supposed to also run reliably and produce good power at less money.

BUT! If the K&B runs okay for you, stick with it for now. Wait until you learn more before you invest. You may move on to a second plane quickly and want a 60 size. Who knows.

Your next step should be to find a local club and an instructor. You don't want to try flying on your own, it's much more difficult than anyone ever expects.

This site can help you locate clubs in your area, and most clubs have instructors who will teach you without cost.

http://modelaircraft.org/templates/ama/

Dennis-