The 1400 phones do System Programming differently from a Partner 18-D - you'll be moving through menus, scrolling down, selecting, changing, back, etc. Not the old familiar Feature 00, Left intercom twice, #504, etc.
The GUI is not bad, just a lot of mousing and clicking.
Be sure to do the 6.0(14) maintenance release upgrade
Fill out a regular Partner planner to get the basic info. Remember that the extension numbers are fixed, so a combo card will have Ext. 10 - 15 as digital sets, then 16 & 17 as Analog sets (if it is in the first bay).
Think about the layout of the buttons, to get the most effective use of them. Redial is a fixed button on the set, so you don't need a programmable one for that. You may want a Voicemail collect button on one of them in addition to the Visual Voicemail button on the phone.