Chum,
Congratulations on taking up the guitar, don't give up, it can be a lifetime joy!
There is no "best" fingerstyle Martin, and you certainly can play any style on any guitar, they just have different strengths and weaknesses, it all comes down to preferences and opinions.
Martin has over 200 models, the only thing there might be "general" consensus over is a Mahogany dread, with the D-18 Authentic, but best anything else will give you 10 different answers from 10 different people. Also budget comes into play, Martin's "best" of any body style can be pretty expensive.
With only a few months under your belt your best move is patience and auditions (coming from someone who started playing in 1966), respectfully you "don't know what you dont know", and we all tend to buy impulse guitars when we first start playing and make mistakes that hit us in the wallet.
I believe Martin's OMs are the most versatile for folks who strum/flatpick and also play fingerstyle and they have the wider nut. Some 000's like the 000-28EC Clapton also have the wider nut, but you're talking a bump in price from your D-16.
If I were your teacher I wouldn't recommend doing anything unless the nut width really is a problem and you know you will concentrate on fingerstyle playing. Just to give two recommendations, but again they jump in price, I would offer the 000-28EC and the OM-21 Special, both wide nut but very different guitars.
Patience. After only a few months you really don't have great mechanics and the D-16 may be fine for you as your skills improve. I play fingerstyle on my dreads at times even though I have small bodies too, work on your skills and audition when you have the chance, take you time..........
