I doubt you would save money buy building your own, I recently build a modest systems for a work collegue and it was almost exactly the same price as similar offerings from Dell & Evesham. You would probably have trouble buying a PC without a monitor from Dell but most other companies let you remove it from the basket.
By building your own you arguable get better parts within your machine because you are specifying everything to your exact needs. Also remember that some PC manufacturers (i.e. Dell) produce their own motherboards and cases that cannot be reused or upgraded.
Also remember you would need to buy an OS software licence if you build your own machine. That can add a significant amont to the cost of your build.
Saying that your budget would allow you to build or buy a very reasonable all round machine with no problems.
I would personally go for the self build option but try to do as much of it yourself as you can, possibly with help from your friend but you might as well treat it as a learning experience.