There is a discussion in the poskim for someone who buys a Sefer Torah if they should say shechayanu or not (perhaps only with a new fruit nearby, etc.) This is seemingly by someone who buys a Sefer Torah already made. Is there any difference for someone who commissions a sofer to write for him davka? Is there any difference by this in regards to making a shechayanu?
I don’t see any reason why there so should be a difference. There is no difference between buying a new dress from the store, or having a dress commissioned, and the same will apply to a Sefer Torah.
For the shehecheyanu blessing itself, see Be’er Heitev (223:10) who writes that many poskim agree that one makes a shehecheyanu over a new Sefer Torah.