I am building a timber frame home (from a 40 x 100 barn)  So I don't need structural panels, only Insulated panels.  The question is this:  I am finding several places on the web that see used Polyisocyanurate panels.  Is there any reason that I shouldn't glue these to 7/16" OSB and I have a very reasonably priced insulated panel??

We have sold recycled and factory second polyiso foam for a number of years.
A big potential problem with gluing polyiso foam that is used is the facers might not be bonded to the foam very well.
When you buy SIPS, there is no facer in between the foam and the OSB.

That being said, somewhere on this site is a thread (in the SIPS section) about DIY panels. We had done them with stick wood and
glue and screws, using the stick wood to sandwich the foam with glue and screws holding the panel together.

OR just shop the regular suppliers for raw panels and save a lot of work.
Remember that if you purchase pre-made panels, the foam is usually pretty inexpensive, so getting an R40+ insulation value
is only a minor amount more than an R20 panel.

You only buy insulation once.