I took my test 3 years ago (in Canada) and bought an older bike - 1983 Suzuki GS650. It's an inline 4 so I can't speak about v-twins (I've ridden a couple but not for long). I love it. Not too heavy, around 70HP so will take a passenger and luggage on the highways with more than enough power to get you out of trouble (and some of our highways are very mountainous). I had a 49cc scooter before that, and despite people taking the piss I still miss it, though I'd probably never ride it if I'd kept it.
1. I'm glad I didn't get a 250cc - everyone I know either sold theirs after one season or is bored of it / frustrated by it now.
2. Personally I'd avoid any 600 sport bike to start with - a lot more power and less weight than my bike even in a 600cc, and they will throw you the **** off if you don't know how to ride them.
3. I'd buy used for my first bike, or just accept the fact that you're going to drop it at least once. The main damage will be to your ego, and any fairings / mirrors / stuff sticking out. I didn't care since my bike is old and scuffed, but I would have been heartbroken if it's been a new bike.
Also, think about how much you want to wrench on them - generally the older the bike, the more work they'll need. Mine's really easy to work on - fluids, filters, and cables are very simple / easy to access, and I've done some electrical and mechanical on it. I was actually annoyed / surprised that you had to take the carbs off to change the starter motor, which is a really unfair criticism but speaks to how easy everything else has been. Make sure you get a shop manual and a Haynes / Clymer manual, and check both against each other (there are errors). My bike lacks things like ABS and has some quirks specific to the make / model so it's not perfect.
The Street Twin is a lovely bike, similar riding position to mine which is great for most stuff. You'll get the wind in your face on the highway but that's half the appeal for me. I don't envy the old boys on Goldwings sat in the armchair seats with the big windshields. Sure, it looks comfy, but I'd rather be in a car at that point. The 250 might feel too small quite quickly, though if you buy it used it'll hold its value for a season and you can sell and step up. The Ducati I've never ridden, and the Guzzi's aren't really my thing. Have you looked at the Ducati Scrambler? If I was starting out I'd take that, the Street Twin and a Bonneville out for a test ride.
As above, obey the two golden rules:
1. Everyone is trying to kill you all the time. You are always in the wrong - not a lot of point being righteous and dead.
2. Wear protection. This means different things to different people, but even if I'm going a couple of kms across town, I'm wearing Kevlar-lined jeans with built in armour, a bike jacket, gloves and boots (for around town I have ones that are basically re-enforced high tops). All of those are a compromise in terms of protection but better than nothing. For longer journeys I'll have proper pants, a thicker jacket, more sturdy boots and even in summer I'll go thicker gloves (my summer gloves are very lightweight).
Two extra rules:
3. If you ride in the rain (I ride all year) no-one can see you at all. Your visor is like a blindfold at slower speeds. Water will get into anything and make you uncomfortable / distracted. Buy your gear (and colour of rain gear) accordingly.
4. No passengers for the first few months. Ideally for the first 3-5000kms, as you will make mistakes. I'm at around 20,000kms, and since the birth of my daughter we've decided that we'll never be on the bike together again and I put my life insurance up. The risk is real.
All that said, it's a ton of fun and hard to beat on a sunny day with your mates. Just take it easy and keep the rubber side down.