Add disk ?

Hi guys,
This is my first post, so please excuse me if I have missed anything or am in the wrong section :P
I have got a DiamondBack Overdrive FX 2011 Hard tail Mountain Bike, admittedly nothing special, but I cant afford a new one at the moment because I have just bought a Giant Defy 1 for traveling to work etc.
What I want to do is two things,
Firstly, Add a disk break to the rear of the bike, the standard V-break is nowhere near as good as the front disk when the going gets muddy! Is this possible to do? how easily and how much?
Second, Is it worth a new, slightly softer, set of suspension forks? considering the bike will be used for very little on road use because that's what the Defy is for
Thanks in advanced
This is my first post, so please excuse me if I have missed anything or am in the wrong section :P
I have got a DiamondBack Overdrive FX 2011 Hard tail Mountain Bike, admittedly nothing special, but I cant afford a new one at the moment because I have just bought a Giant Defy 1 for traveling to work etc.
What I want to do is two things,
Firstly, Add a disk break to the rear of the bike, the standard V-break is nowhere near as good as the front disk when the going gets muddy! Is this possible to do? how easily and how much?
Second, Is it worth a new, slightly softer, set of suspension forks? considering the bike will be used for very little on road use because that's what the Defy is for

Thanks in advanced

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VOODOO CANZO
Come and see me at https://www.facebook.com/biketyke/
http://www.ashcycles.com/site/diamondba ... d01890d935
Should provide more information that I cant remember
As for the disk, I think an adaptor would be in order, there are no screw holes for the caliper on the frame
My budget is dependant on the gain, because I am just getting into MTB I am not experienced enough to know what will do what.
VOODOO CANZO
Come and see me at https://www.facebook.com/biketyke/
Might save up for a Giant Talon or Trance, when money is slightly less scarce
B'Twin Triban 5
Well in that scenario it's not the expense it's the fact the frame can't take it.
retired 9.6kg Carrera Kraken
The Carrera Hardtail combined thread - come on all you Carrera's!
The Sons Scott Genius RC20 build
Ribble Sportive 7005
the rear brake isn't overly useful or effective anyway.