This is an awesome forum and pleased to meet you here.
I have done the Mousseline Buttercream many many times now. All except once I used unfrozen egg whites. That batch did not emulsify even after 15mins of vigorous whisking. I was afraid that my beater would died on me so I chucked the entire bowl into my freezer. Waited for another 15mins before I subject it to high whisking again. It finally pulled its act together like a chided child given a lolly. I hope with my explanation you see how important that the temperature of the whites and the temperature of the butter must be at least same or thereabout.
As Hector puts it..he always flavoured his mousseline. I find that my current favorite is adding lemon curd (Rose’s Lemon Curd, of course!) to my mousseline and I flew to heaven (oh, you get what i mean..).
As for your second and third questions on the digital thermometer, I use any unsalted butter I chanced upon and as for the thermometer, you can most certainly get it from Bake with Yen or Chang Tung for less than RM80. It is essential that you lay your hands on one before you attempt this challenging buttercream. All the best!