You can restrict a 500, this would be worth doing as you can reverse the process when you want/are legal to have more power. If you buy a new Honda the kit is free (about £150) and fitting will be around £50. Kits can be fitted to used bikes, but there is a little more cost, but you will pay less for a used bike anyway!.
One thing to note above all else is that you must get a certificate stating that your bike has been restricted. There are unscrupulous dealers/privateers that will supply just the certificate i.e. without restricting the bike DO NOT FALL FOR THIS ONE! as 1. Your insurance will not be valid and 2. You will get a big time fine and points etc.
What to get a 250 or 500? really depends on what you are using the bike for (commuting etc) and how long you intend to keep the bike. If you want to keep it a couple of years or so after going to full power I would suggest the 500 (they are more saleable aswell) though the 250 is quite adequate as a commuter!?!
With a new bike you get a 2 year warranty (restricting the bike if done by a qualified person does not invalidate your warranty, and you get a good deal on the kit from a Honda dealer) i.e. New CBF500 ABS model is £3,999, plus £50 fitting of the kit.
Or consider a used bike, but make sure you check 1. Service history 2. Correct mileage (verify) 3. Number of owners. 4. Whether it has been a write off or outstanding finance 5. Import (worth £300 less on average) etc - all of which you can check via MCN/Biketrader etc. Golden rule with used bikes is, if in doubt when buying, walk away, there will be plenty more
Sorry to go on a bit, but if you are not careful you can easily get ripped off when you are new to bikes and buying used.