0.4ºC System Accuracy is ok, but nothing special
a $50 Fluke 2190A off eBay will do better (+/-0.22ºC to 90 days from calibration with a type "T" TC)
- but to get it to below 0.1ºC is much more expensive, different gear altogether
"that the 946 can upload to a pc" is certainly useful;
the IEEE 488 business is a PITA, and expensive for the hardware and software
some basic info on measurement uncertainty is
here
getting a number is not so difficult, also with lots of zeros
understanding - and demonstrating - the validity of that number can be taxing
(for the non-statistician)