As soon as you are in a jam session and someone says "Let's do this one in A", you will want the 5th string capo or spikes. That plus a regular capo makes it possible to play in keys like A and B while playing "as if" in G. Very useful. I recommend your getting hold of my video Bluegrass Jamming, which will give you a chance to jam "safely" in private and get used to the skills you'll need, including playing in keys other than G.

D-tuners are ingenious devices to allow retuning while playing, or quick retuning to a particular (D) tuning between songs. The repertoire that uses them is not very big, and in reality, d-tuners don't get a lot of use from most pickers. Not at all a necessity for a novice picker.

Pete