I haven't even found Buzzer's shop. I went south from Rail Crossing, saw enemies, and turned right around again. I really expected Tunnel Hatch to be right in town so I could bypass all of that by going underground.
Buzzer shop is IN Rail crossing, transfer through redzone on left of Captain's office. There is a bypass route, through buildings, where you can avoid plasma sentry, also there is a way to disable them - quest for it is given by mechanic in shop. And, that tunnels in which you try to get - filled with burrowers, be ready for them.
For more details read underrail official wiki.
ALways remember - scout area prior to combat action, do not just run away.