Yeah the are a pretty good bike for people starting off with pitbikes.
Are you thinking about the Mid Size Nitrous ?
They have a longer wheelbase and the seat height is higher
You will need to upgrade the shock and forks if you are wanting to jump it, but general use they last a while
Have you looked at the Fuse 125 too, they feel very strong too, i might be biased because i used to own one.
But i don't think they sell them anymore ?
Also there is the Atomik Comp L race bike , they'd be a lot better for trails etc due to the Linkage type rear suspension and adjustable forks
Not really worth fitting the exhaust on a 125cc, from memory they come with a 32mm big bore system standard?
A new 730mm GPX fork kit is $400-http://www.ebay.com.au/itm/NEW-GPX-MOTO-MINICROSS-BLACK-LABEL-FORK-KIT-730MM-KIT-/331083592961?pt=AU_Motorcycle_Parts_Accessories&hash=item4d161eb501
A good rear shock would be between $100 and $200, maybe more.
You might be better off with something like this, they look ok for under $1000 Assassin Dirt Bike A9 BIG Wheel Foot 14'' 17'' Rims YX BIG Bore PIT Bike Bike | eBay
The bigger wheels will be easier to negotiate trails/tracks etc.
Its has a bigger engine so more torque.
And Change out the fork oil for some better quality stuff, or just ride it till the suspension fails then fit some better forks/shock like the DNM's
Or look around at a used pit bike, there have been some killer bargains on here lately