hello bro, for office documents, files management and apps usage iPad Air is sufficient bro. consider Pro model only if you need that OLED display for any photo, video editing color correction purposes and budget isnt a problem.
Cellular model is suitable if you want to put separate sim card to use internet while roaming. if you like to use hotspot from iPhone then you may go with wifi model. or in most places you stay has wifi connection, then wifi model is sufficient. else cellular can be considered.
12.9 inch model with mini LED Display also best buy right now bro. performance difference heavy tasks la mattum thaan notice panna mudiyumnga. for office purposes you wont see big difference.
Budget wise if you can find 12.9 inch with good deals and discounts then consider it.
Yes bro. 3 to 4 months aagumnga. irukara stocks sellers valiya sell pannuvaanga. online and offline shops. trusted shops pathu vaangalaamnga. 12.9 is good model to consider bro.