I guess it all depends on how responsible of an adult you are? I put a few years in on a RM125, stroked it to 151, and gave it to a friend after awhile. I'd spent about 1hr on sport bikes during that time... And then four years of no riding anything later I bought my first street bike - a brand new off the showroom floor Buell 1125CR. It's a lot of bike, but I don't go around riding like a moron 99% of the time. I was pretty timid on it at first, but feel I know it better now (2500miles, bought it in November). It's seen rain, night, day, fog, etc. Riding in the rain is...pucker factor 9. I test rode a CBR600 RR for a friend in February - slower, but still fast. Definitely not the accidental wheelie machine that the 1125CR is. Honestly loved the ride of the RR and I don't think it's a prone to biting people as I'm told as I felt very comfortable on it and it seemed forgiving, more than the Buell at least. I rode it for 30-40 minutes at a decent pace, too.
I have two unintentional wheelies (0 on purpose) under my belt. Sky high at 75 will make you rethink certain things in life... I'm getting 35mpg overall with 90% being in town/traffic - the majority of my riding is under/at 5k RPM and 1/4 throttle. Real time says mid-50s highway, and 40s in town. The real time is probably close to actual considering idle time and a whack of the throttle now and then. I love the 1125CR. It's different, it's reliable, it was cheap due to HD's brilliance (haha), and I'll never outgrow it. There is way more bike than rider to the combination. I'll be lucky to ride it to 8/10s of the bike's ability...hell 7/10s...5/10s even probably. It's wicked fast and corners amazingly well. I will track it at some point for curiosities sake.
In general, to me:
1. For long highway rides, a front fairing is pretty much a requirement. I really hate mine after riding the RR. I plan on adding a windscreen or converting my bike to use the 1125R front. Laying on the tank @ 75-80 for any distance sucks and doesn't help that much as I still get loads of buffeting. The RR @ 80 feels like mine @ 30...sitting up in the seat on both. Tucked and it's more like the RR @ infinity vs. mine @ 0mph. Wind sucks.
2. Torque is handy, but too much is bad. The peaky powerband of the RR (inline 4) would probably not be much fun to DD. I do a lot of city riding (sub 50mph), so it's nice to put around. A V-twin is great for this. Tractable, a little harder to kill from a stop, etc. Mine makes for an easy wheelie, however - V-twin downside. 1st gear is no man's land with anything over 3/8 throttle. Even laying on the tank it can be hard to keep the nose down in 2nd.
3. The MSF folks are adding some new additional courses in bigger cities at some point. If they are already offered and you have the opportunity, take them too. I had to unlearn a LOT of dirtbike habits for the street and the basic MSF course was a boon of info in that regard. It's a good start. The newer advanced courses should be a big benefit.
4. ABS would be nice. I don't have it. I would like it after locking down the rear once (didn't let off of course) - cold tires + cold brakes + cold road = hard to panic stop.
Pro tips:
1. Respect your machine. A Ninja 250 will kill you just as fast as an R1.
2. Everyone is trying to kill you/You're invisible. It's one or the other. If they look directly at you pulling on to street/turn lane, THEY CAN'T SEE YOU! Never, never forget that. No one sees bikes. I ride with my brights on (daytime). NO ONE SEES YOU. EVER.
3. Also, mirrors. Use them.
4. Look by turning your head before a lane change. Your mirrors lied to you about that empty lane.
5. Tailgating is stupid. Don't be stupid.
6. ATGATT - All The Gear All The Time. If you have a Shoei head, get the RF1000 (older) or the 1100 (new). I have a TZR, the extra aero bits are worth the cost on the RF.
If you have an Arai head, ask someone that wears one and get an equivalent. I have ICON gear because it's the best of what I could afford at the time. It's textile so it'll probably last for at least 1 wreck. Mesh will probably be 1 and done. Leather is best. I have the ARC jacket and pants. If you have a jeans workplace, get some A* 1.5 denim jeans. You can add kneepads and they look good enough to work in. I would like something with hip pads, but no one makes what I want. I run all the padding in the jacket plus an Icon Field Armor vest. Dainese has a very nice looking piece of armor, the V Neck Wave jacket. When I upgrade to leathers I'm also getting that to replace my FA vest. My boots are Sidi and my gloves are ICON full length with CF knuckles. I wear a load of gear, I'm in Texas, it's hot, I don't care. I want my survival odds as high as possible. I recommend that you get some combination of gear that makes you feel comfortable and feel safe. I have an ICON MilSpec yellow vest I wear in the mornings most of the time. It is bright/reflective as the damn sun...People still DO NOT SEE ME! Do what you want, but I'd like to be around for a long time.
7. Earplugs. Save your hearing, You lose some local hearing of cars. I still hear horns, sirens, screeching tires, etc. I can't hear cars next to me, but with my bike (very loud stock), I couldn't anyways. The lack of earplugs makes me feel like sensory overload.
8. Small shifts in weight, throttle application, hand/wrist movement, clutch/brake application can make big attitude shifts in the bike. Be gentle...it's your wife, not the 20yr mistress.

Unless that's your wife and then, well, hell...
9. Head in the game man, head in the game. Make your brain work for you on the ride. Zone out and eat a curb. Bad things... Guaranteed you'll be concentrating more than anyone in a car around you. Situational awareness is a lifesaver.
I would say an SV650S would be great to start. V-twin, tons of info on forums, relatively cheap maintenance, aftermarket parts - susp/power/gear (bags and other farkles), reliable, easy to work on. Ride it slow, respect the hell out of the bike. I was just going to say SV650S and then all this crap got typed out. Sorry...
PS: NO ONE SEES YOU!