Personally I wouldn't knock the second hand market if you look at reputable sellers. On a lot of club websites you can often get second hand bikes for a lot less than you would pay retail. What I would say is make sure you have an idea of what you want (infact with buying any bike this is key, and also what you want to do with it) and you know a bit about what the seller is selling, plus give it a good once over. If you could get someone who knows a fair bit about the mechanics of bikes and what's value for money it doesn't hurt. Buy complete, don't buy parts because often building up can end up costing more than buying complete.

It may even be that your LBS might have in something second hand that someone has part exchanged, which is even better because they will have given it the once over. As said before if going new look at your LBS.

Knowledge is the key to know what you are buying and get value for money. Scout around, and sometimes if you see something you like at your LBS and can see it cheaper online speak to them and they may be able to do a deal on it to get your custom. Personally I would never object to paying a little more to get something from an LBS as they will often give you help and support which you would rarely get from an online retailer.

If only one bike is an option and you have a feeling you might like to do a mixture of a bit of road and offroad trails look at Cyclocross as an option with a couple of sets of wheels depending on what you are riding (though you can get crossover tyres which give you a bit of both worlds. Often the cheaper good value buy will do great as a winter bike if you feel you want to get more involved and spend more.... TBH I went mad on bikes a fair few years back, but then being away means I have barely ridden any of them as I've lost my fitness and don't have space. So at the moment my cross/winter bike built up with cascaded parts from other road bikes and a few extras added in sees far more action as I often use it for the commute.

With regards to the disk option, I've got mini V's on my cross/winter bike and they do fine (everything else is a road campag groupset), but I do regret that I haven't got the option for disks because they would actually make it a lot easier to swap the wheels around - there are issues with clearance with the mini vs which mean when using cross tyres the mudguards have to come off as they foul the cable on the V's (with a disk system that problem is eliminated and you could probable even get away with setting the mudguards a bit higher to give clearance for the cross wheel tyres). That said I think you can get cable pull devices which would allow you to use the full size v brakes with a road shifter which might also go some way towards resolving the problem, but I still think if I was choosing now I would go for the disk option rather than the miniv's.