Agree go with .272 no neck turn standard br for first gun. Just get the reamer with enough freebore for the 100-108 gr Boattail bullets. I would order my own reamer from Dave Kiff at Pacific Tool and Guage for $140
Go with a Broughton 5C barrel finished at 28 inches. No need to go longer and the 5c canted rifling is definitely 50-75 fps faster.
Try the CCI standard and magnum primers, lapua brass and either VARGET, RL15, or VVN540 powders. Bullets either the SMKs sorted or berger 105s. Best IMO is the Clinch Rivers but good luck getting any. James Phillips on www.6mmbr.com
often has some for sale ($360 per 1000) as he shoots a lot with Greg from CR a lot.
I shot a short neck BR improved and now went to the 6mm BR DX which is Ack Imp with long neck. Same capacity as Dasher with longer neck. However, no standard dies available all custom.
Stocks pick your type.