The 'Classical' set cannot be tuned down to D. IMO if you're building a minstrel banjo, put minstrel strings on it, you'll be playing in G and D. Playing solo, like you said, you'll WANT the full sounding lower minstrel strings. It's a world of difference between the high sound and the low sound. People love the low minstrel tuning, it's different. You can play with others who can play in G and D which are very common keys. Having played other instruments, you'll find the minstrel tunes a challenge at first, then when the technique (stroke style) soaks in, it's really quite easy. Very easy physically. You don't need callouses, you NEVER get fatiqued from fighting metal strings, and there are 'reaches' involved when you are playing the guitar and fiddle that wear you out. The banjo is light, thin, easy to play and doesn't fight you. Kindly tell your guitar playing friends to concentrate on their bass notes and strum easy or they'll cover you up. I don't know your musician friends, but many guitar players like to over-do it, like when they sense your DUN da da rhythm, many modern guitarists start with the JING jing-a JING. Don't let it get you down. Accompanying minstrel banjo is tricky. Get some recordings and have them listen and learn.
Sawmill tuning is gDGCD, also known as G modal. The minstrel strings may not go that high, plus we play a lot of those modal songs capoed or tuned up to the key of A, essentially aEADE. You can get an equivalent tuning from the minstrel strings by raising the second string 1/2 step from the minstrel tuning. But this won't put you in the key of A. To play with more modern fiddlers, etc, you almost need a second banjo strung for the G tuning. I haven't experimented with using alternate tunings with non metallic strings, but I suspect it would be hard to keep them in tune if you change tunings frequently. When I change my nylon guitar or ukulele strings, it takes at least a few days of stretching before they will hold tuning long enough to play out. Rather than re tuning, or capoing, it may be possible to move your bridge when you need a new tuning. This won't give you a lot of flexibility, but might let you go up a tone, say from D to E. Seems I've seen someone post about this.
Question: Can Nylgut Minstrel strings safely be tuned up to “E”? eBEG#B.