Hey there. New to the whole whistle thing, so I’ve got a couple of questions. Is it preferable to get a tuneable whistle? Also, I’m thinking about getting a low whistle but am unsure whether to get a G or a D (it would be my first). Is there a large advantage to getting a D as far as range is concerned, or can a person get by just fine with a G?
Whether your first whistles are tunable or not, you will be buying more eventually, so just get a few cheapies.
Get Ds, Cs, and Bbs at first, then move on to As, Gs, and Fs, and low Ds after a little while longer.
www.thewhistleshop.com is a good place to start.
I concur with Cranberry (interesting new avatar, Bud…)
To give a direct answer to your question: Starting with a low D may be counter-productive. There is no such thing as an ‘easy’ one… A high D is the standard of the ‘industry.’ If you absolutely must begin with a low whistle, either a ‘kind-of-low’ A or your mentioned G would be a better starting place than the low D. The A will actually allow you to play in the key of D, a most popular key for folk music…
Tunable instruments are more expensive, but allow you to play with others, more easily…If this is an issue for you, go tunable. A nice high D tunable instrument is Mack Hoover’s Brass D Whitecap. It will set you back $45.
There are of course many, many whistles that are less expensive…However, unless tweaked by a person with experience, 'ye gets what 'ye gets.
Best to you and welcome.
Byll
I agree with both of the above, and I would like to add that in addition to a high D, a high C (the next step down) whistle could be included. It’ll give you an idea of the differences between the two tunings (regarding breath control and intonation) and it will also give you another scale to break the boredom of D with. 
The others have shared their wisdoms with you, so I shall just say hi.
Yo.
As far a key goes, if you don’t plan to play with others (or to play along with instructional CDs), that reduces the need to start with a D.
I have a Hoover A Whitecap on a 3/4" aluminum tube that is easy to play–both in terms of breath/octave changing and fingering. http://home.bresnan.net/~mackhoover/ordering.htm (All Whitecaps are tunable.)
You also might like to try the Jubilee practice low D, which has pretty easy fingering for a low D. http://jubileeinstruments.messianic-webhosting.com/pr-lowdrev.htm At $22.00, it’s really a good deal. The older model took a lot of air, but I understand that the new one is better. (It’s not tunable, but the one I have is pretty well in tune.)
I believe that all of the inexpensive whistles with plastic heads on metal tubes can be made tunable. There are threads on this. I’ve made an Oak and a Feadog tunable by the application of a little hot water.
If you go for one of the high D cheapies, be aware that quality is not consistent. In my opinion it’s better for a beginner to spend a little more on a Jerry Freeman-tweaked Generation or a Hoover Whitecap than to fight an uncooperative whistle, not knowing whether you’re doing something wrong or whether it really is the whistle.
That’s true. I started on a Walton with an absolutely rotten 2nd octave and thought that it was me (actually I think a couple people on the board said it definately was me 
). So I put off playing for the summer. Then a ran into a MEG while in Halifax and picked it up just for the heck of it. All of a sudden I could play the tunes that continued to get massacred on my Walton.