If money was not the issue here, as a D35 will be quite more affordable than a D50 or even a G37 for that matter, I would start with the D50. It is your quintessential bluegrass dreadnaught. Super loud and can carry many a songwriter/singer at a solo gig. LIke the Martin D-28 but louder and more evenness of the volume across all strings. Don't get me wrong, I love me a good D-28. But a Guild D50 is just grittier. Meaner...
I have owned 4 D35s, they are rather standard ho-hum spruce/hog all solid wood dreads. Real clear and a good all around workhorse but will not have the sustain of a D50 or power.
G37 is spruce top and maple b/s but an arched back so should be some good volume output and be brighter in sound vs the D50 and D35. I think the G37 is the most unique of the 3 old Guild dreads, as the D35 and D50 are not really "unique" but IMHO, just great all around standard dreads that most other old guitar mfgs make. Guild just does something better with their builds re volume output vs other mfgs.
So all 3 solid wood dreads, but the back of the G37 would be laminate
D50--Spruce/Rosewood cannon
D35--Spruce/Mahogany workhorse
G37--Spruce/Maple brighter, more clarity, probably better dread for fingerstyle.
My first buy would be the D50.
The order of the other 2 would not matter...
Nice collection either way!