Monster would make a good bike for a first big bike and I would say that a 620 would be the best choice on your budget.
As with most things, get the latest one you can afford - the 620 with the APTC clutch, introduced about a year or so ago has a much lighter clutch action than the earlier bikes clutch so could be a good choice, if they are within budget (although I suspect they'll be upwards of £3K).
Whatever bike you get, make sure it's been serviced properly and remember that higher mileage on a well maintained bike is not a bad thing. In fact it's better than a bike that has been sitting around.
Make sure the cam belts are within their service life, they should be replaced every 12K miles or every 2 years.
As far as servicing goes, with a £2.5K bike the chances are it'll be more than 2 years old and out of manufacturers warranty, so a good independant will be fine
Jools
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