Choosing a long-travel cross-country fork
Longer travel will change the bikes geometry. A 140mm travel fork has 60mm more travel, therefore needs 60mm more clearance. To get this, you need a smaller wheel or longer forks. Presuming you are sticking to the same wheels, you have to raise the sheerer height. The effect of this is a more relaxed sheerer angle - changing the handling for the bike. 

Personally I would not do it. The Hardrock is not a down hill bike, and never will be. With your target price, as you have observed - you get a lower travel higher quality fork.  This would be far better. The fork will be stiffer and more robust, probably lighter and likely to be more adjustable. 

In your situation, I would put on the highest quality shock I could afford, and forget about increasing travel. I recommend 100mm to 120mm max. A well setup better quality short travel fork will walk all over a cheap long travel fork on all but the most extreme DH rides, and the compromise when not on those extreme DH rides is, well, extreme......