hey = hello
we = 1st person plural
are = doing
finishing = getting to the end stages of something
a Reno = a renovation of a property
and need = not just want but have to have to complete the Reno (see Reno above)
a fair quantity = a lot
of 10ml sheets = Sheetrock wall board 10mm thickness
who = which person (in this case which builders merchant)
is the cheapest = can I spend the least with on the products we (see we above) need (see need above)
around = in the area or neighbourhood
the Mornington Peninsula = part of Southern coast of Australia in the Melbourne harbour region known for its hot springs