At 6-2, 230 lbs the top heavy nature of the DL650 shouldn't be a problem at all, even for a somewhat new rider. You are aware of the condition and will handle the bike accordingly at slow speeds and while moving the bike around in the garage and parking lots. I don't really notice the top heavy nature of my DL1000 once I am moving faster than probubly 15mph or so, and it weighs about 50 lbs more than the 650. I only notice the weight at all when traveling at a crawl or moving the bike manually so don't worry aboout it to much.
Engine guards are essentially a bolt on cage that attaches to the frame and protects the engine and gas tank somewhat from damage from simple drops and milder impacts. An engine guard will probubly not be the deciding factor in whether the bike would be considered "totaled" by an insurance company in the event of a significant crash or lay down/slide but if you drop the bike at a light (ooohhhh, never done that... so emberasing) or in your driveway in front of your friends or sweetie (doohhhh!!!), then the cage/engine guard will take the hit and you won't scratch things up or poke an expensive hole in your fairing or gas tank. Allot of people put engine guards on, they don't detract from the looks of the bike at all (in my opinion). Scan some of the ride report threads and you will see that most strommers have some sort of engine guard on their bikes or link to amotostuff.com and you can see pics of some engine guards for the Strom (I don't work there).
For the sore butt issue, many DL650 owners swap out there stock seat for a DL1000 seat and find that quite a bit more comfy. The DL1000 seat is 1 inch taller so only do this if you have the leg room, and at 6-2 I would assume that you would have the leg length or you are a genetic mutant with an unnaturally long torso and short stubby little legs.
Best of luck, the DL650 is a nice bike and will very likely give you years of trouble free service and miles of smiles. Take it easy for the first couple of months, practice low speed manuvers in a large parking lot, and maybe some panic stops, and then some higher speed breaking and by the time summer is here, you won't be a newb anymore