a good idea
here is a slight addition to the idea
pop along to any PVCu supplier or even a glass cutting company , tell them what you are planning to do, offer a service to your customers for changing broken units
they will tell you what kit you need
off the top of my head it will consist of
a glass thickness measurer ( ask them to explain how it works )
a set of Allen/Hexagon Keys
a mallet
a pair of plastic trim removers
thats about it i think
i cordless screwdriver is handy aswell
now its easy enough to measure the glass, ask the glazier how much is the norm to subtract from the window opening size ( you will see the edge of the window / start of the window trim bead i allways used to take off 10 mm top and width, this gave you enough room to get the glass in and pack with pvc spacers
measure THREE time to check

measure unit, inc thickness, the little tool will tell you how thick it is
take it along to a glass cutting place, place order, they will phone when the unit it made ( norm 10 days )
take unit to customers house
remove beads ( on older windows they had a rubber gasket / bead ) remove this first, then the bead, on todays its just a internal bead
take out broekn unit
fit new unit, pack gaps around the unit, so the unit doesnt move ( normally each corner top and bottom and then side corners) get a big bag of wedge shaped plastic packers from the suppiers of your unit, they come in different colours ( which are different thickness in mm )
tap beads back in
walk away
oh hang on, i should sell this as a e-book
