Hello,
I like your question, and it is one that I have faced myself. I have bought two fully built up bikes in the last two years, and also a frame and then added the parts to make a complete bike. It is my experience that you save a lot of money buying a bike that is already built up by the manufacturer rather than doing it yourself. This is because the manufacturer gets a huge discount on every part on the bike compared to what you, or I, or probably even the local bike shop pays. So it is easy for them to sell complete bikes that are way cheaper than a person buying each part themselves.
That is not to say starting with a bare frame is a bad thing. Doing it that way gives you exactly the bike you want, but, in my experience) you will end up paying extra (a lot extra I think) for it.
If your looking to save money on a do it all, on and off road bike, I think that the REI Novara Safari is something you should consider. It is a hard-tail rigid steel fork 26" wheeled aluminum framed mountain bike with treking bars and comes with a rear rack and disc brakes. List price is $849 here in the USA.
www.rei.comIf you want to build up similar (though all steel) bike yourself, I would suggest you start with a Soma Groove frame (list $375.99 US) which comes ready for discs or cantis and is lugged for a rear rack. It is also designed for 26" wheels, which some prefer for their toughness and the easy of getting tires and tubes anywhere. The matching rigid fork is $119.99 (with lower lugs only). The frame and fork are a deal for the quality you get.
www.somafab.com But as you can see, your already up to almost $500 dollars and that is before wheels, brakes, gruppo, saddle, etc. No-way your going to come in under a thousand here.
However, this is the route (Soma) I went because I already had a lot of the parts from my old mountain bike, and had some pretty particular ideas about what I wanted in a bike. However, I ended up spending over a grand. I do love the bike though, which I use for commuting/trails/touring.
So, there ya go.
Hope that helps,
Gregg