As to brands, the nicer Yamahas are nice, Malagutis are nice, Aprilias are very nice (I've owned two) and pretty fast, Hondas are nice, Vespas are well built but slow, Piaggio and Puegeo are nice but hard to find sometimes, and Derby's are OK. I'd avoid any chinese make, since they just aren't up to par right now. Japanese and Italian are generally good choices. If you plan on 'hopping it up' a bunch, look for a carb'd model, but know that they take a lot of work to make fast, but have a lot of potential. They also get worse gas mileage, and make more pollution. Fuel injected versions, like the Aprilia Ditechs are typically faster in stock form than the carb'd models (especially when software derestricted), but can't be tuned up to the same level. They are cleaner, and get better mileage, and tend to just run a little better. They are also more expensive.
peace,
sam