Im a motorbike mechanic and have a few years experience with scooters + own and ride four scooters myself.
For your first one, stay away from chinese. A decent used KYMCO, Honda, Suzuki etc will set you back £400-£500 for a 50cc and £600-£800 for a 125cc. Unless your good with mechanical stuff, id stay away from anything below that price range, they tend to be sold my young lads who have ragged them to within a inch of their lifes and are about to get mums old citroen saxo and need the money for a big baked bean can exhaust. However if you are good with wrenches and can work your way around a simple engine, you can get some pretty good bargains. (i picked up a Honda Vision 110 for £150 a few weeks back that needed rollers and drive belt)
Insurance for your first year will be high but does drop once you have a years no claims, Ive been riding four years, I pay £120ish fully comp a year on my Kymco Agility 50, £110ish a year fully comp on my pulse scout 50, £160 a year fully comp on my ZZR400 and £180 a year fully comp on my Tiger800. I was paying £330ish a year in my first year on a 50cc and the same for a 125cc in my second year, insurance on 50s and 125s now comes up at roughly the same now ive got a few years no claims.
I say stay away from chinese bikes if your not good with mechanical stuff because they need ALOT of maintenance. They are OK for short distance commutes but thats about it. Thats what they are built for. If you want something that will last, go KYMCO or jap.
CBT is compulsory but isnt if you passed your driving test before a certain date (cant remember what it is but i think its sept 2001) however its still a good idea to go and do one, youll learn alot. Its fairly easy and takes about a day. Once you have the bug though youll want to do your full test and get on the big boy bikes and ride with the big boys
Scooters and bikes are a great and cheap way of getting around but dont forget how dangerous they can be, Get the proper training and wear the right gear, a good jacket, a good helmet (the £30 ones off
ebay will do jack sh** when you crash, ask me how i know) and a good pair of gloves at the mininum.
If its going to cost the same as a small car, honestly, despite how much i love riding id say go for the car. Alot more practical, protected from the weather
and alot more comfortable
EDIT - Just to add, I currently have 3 chinese bikes in the workshop, one of them being a modenna, less than a year old and it already needs front wheel bearings, variator, carb, rear light housing and headstock bearings. The other two are two years old and need alot more work, one is having a section of the frame welded where its rusted away and left a hole.