Here's a thought I had recently regarding this exact "kit".....
While 18" raduis curves are tight for most HO equipment (especially modern stuff) it's perfect for narrow gauge. In looking at the Grand Valley set-up it lends itself well to a small HOn3 layout. With the new HOn3 stuff coming out from Blackstone (the K-27 'Mudhen' is awesome) and Micro Trains you could have a pretty sweet set-up.
I've even toyed with the idea of doing dual gauge trackage. Of course you have to modify the track plan to eliminate the 90 and 60 deg crossovers since these are not available in dual scale. You would also have to modify 1 narrow gauge turnout to have and extra outside rail on the straight thru portion. (this means only narrow gauge stuff can use the siding) but then you can run HO and HOn3 trains. If you run standard HO stick with smaller cars (40' max box cars), small diesels (RS-2/3) and small steam switchers (0-6-0, 2-4-0, etc.) for more realism.
Now I know narrow gauge isn't for everyone but going this route allows a little more realistic look to the rolling stock. Narrow gauge freaight cars are 30' long and would look much better on those tight radii. You're going to pay a bit more for the rolling stock and loco but that's a decision you have to make, which IMO would be worth it. You could even run narrow gauge passenger cars with out them looking funny on the tight curves. The good news is structure kits are plentiful since HO is HO whether narrow or standard gauge.
You'll have to evaluate what future plans might be. It's possible to integrate the Grand Valley into a larger layout but IMO that would be harder to do than starting with a blank sheet. No matter what you decide (HO or HOn3), as others have said, allow for at least an expansion module (akin to what John Olsen did for his Jerome and Southwestern).
The Grand Valley looks like a very quick and easy way to "jump start" a layout and get trains running. One of the easiest ways for a newcomer to lose interest in the hobby is to spend a lot of time doing the prep work before actually running trains. The faster you can get to the point of running a train will make you feel like you've accomplished something and begin to get enjoyment from it.
just my nickel's worth...
-G-