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The latch is not required, but it does help with opening and closing the idler chamber during a print. The spacer on the other hand is somewhat required. It goes between the extruder body and the extruder mount and allows the upper bolt to tightly secure the extruder body to the upper mounting hole, increasing rigidity. Without that spacer you either can't get it tight enough, or attempting to do so would crack the extruder body. I usually glue the spacer to the back side with something like plastruct bondene glue.
To build this extruder, it's essentially identical to assembling the stock lulzbot extruder except for the taller mount piece. Instructions for building the stock extruder are here:https://ohai.lulzbot.com/project/3869b291-4dcf-4467-956e-ea37c91cffe9/hot-end-tool-head-assembly/
and the hot end itself is here:https://ohai.lulzbot.com/project/cf3b0638-1e1e-4bff-a00e-c3308d914c68/hot-end-tool-head-assembly/
The differences are that you would want to purchase a 1.75mm hexagon hotend body with a 1.75mm size .3mm nozzle. You want to have the filament as closely constrained as possible until it reaches the nozzle tip. You could use the 1.75mm body with a 3.00mm hotend, but its going to have significant retraction issues if you do.