I would attempt to find the existing line if you can.

code will dictate depth etc

DONT use a chimney, buy a high efficency furnace that PVC *vents thru
the wall goodman furnaces are low cost and work fine

much more efficent

We had considered this for our garage, but decided not to because the
furnace wouldn't be used every day, and we were concerned that the
condensate would freeze inside the furnace and possibly damage the
furnace internally. I know that you can install heat tape and make
other provisions to keep it from freezing, but the additional 10-15%
efficiency over a standard vent furnace just didn't seem to be worth
it.

JK