Honda CBF 125 security, safety and price query?

I was interested in buying a Honda CBF 125 as my first learner bike. I have looked around and found a few for sale (second hand) for about 1.5k in apparent "good condition" and an average 6k miles. Would it be better to get a cheaper bike with more miles or pay £400-500 and purchase one with less (about 1-2k miles)? Additionally, what is an affordable (within £100 budget) disk lock that can give adequate protection to the bike which also fits?

Honestly there are cases where it would be better to go for the bike with less mileage, and cases where it would be better to go for more. It depends how well the bike has been kept, you should definitely look at the bikes before buying, and you should look for a few key things, including how much rust there's, particularly on the exhaust as 125s and really a lot of bikes in general are notorious for having fast-rusting exhaust pipes. Sometimes a bike with more mileage will be maintained and kept a lot better than the one with the lower mileage. You should also witness a cold-start, meaning the engine should be cold before the seller starts it. Don't let them start the bike when it's already been warmed up as it could mask potential faults with the bike that will stay hidden until you've had it sitting overnight. Also check out Xena disc locks, they're pretty good and will definitely fit on the brakes of your bike.