CB-1 - Really nice ride w/ baby CBR engine. I've heard rumours (ahem) of certain garages messing with the carburation to make them run like shit, dyno-ing them to get the 33bhp certificate printout (req'd by the ins?) and then just putting everything back! I even heard of this being done to a Bros 650!
Bros 400 - lovely bike. Compact, easy to ride (and the chassis will easily outperform even a quite good rider, so you won't be limited and desperate to move on once your 2 yrs is up). Lovely v-twin power delivery too.
SRX 400. Same as above really, but single. May be kickstart only (? can't remember) but they look really nice. Few spare parts - lots of back street dealers tart them up and sell them for way over book.
I u look around u can get any one of the above for under 1500 notes easy (bros for under a grand) and second hand bits are cheap if you're around london. It always pays riding something used in the courier trade. All the bits thatbreak are easily available. IMHO the above bikes are all pretty reliable too (despite being greys).
Best bet? A newly imported Bros 4 with low mileage from a good dealer / importer for 1500 quid. New tyres, pads and a thorough carb clean shouldbe included. And you might get a free service too.
I know all this cos I was in your position. HTH