I have an old Brother...526LCW and an old Singer...14U44. I've never had problems threading either of them. I never could understand the fear of threading a serger. I just use tweezers & dental floss threaders (for the loopers)....which by the way, they're now selling in fabric stores as serger threaders. Buy them at the drugstore...much cheaper! As long as you thread it in the proper sequence, you'll do fine. Most (if not all) sergers have a threading guide inside the door. I can thread either of mine in 5 minutes or less.
The price difference between an air-threading serger & one you thread yourself ridiculous. Up until this year, Babylock was the only serger with the air-threading feature. Their patent ran out this year & I believe Juki is now making one.
As far as buying parts (loopers, needle plates, etc), Singer would be your best choice in an older serger. Most parts are still being made. For durability, both of mine have seen HEAVY use & still run like new. The Brother, I bought new probably in the mid 90's. The Singer is approximately the same age & I bought it used.
Differential feed is handy though neither of mine have it. If your wife is very good at handling fabric, managing presser foot pressure, etc., she could more than likely get by without it. But if you found a good used one with it...that's better than without! I was used to using a factory serger without it so maybe that's why I've not had a problem without it.
Both of mine use easy-to-find industrial needles...most new sergers use household needles. Singer packages the ones my Singer uses but, I still buy Organs on ebay for both....about $20 for 100.
I'd also recommend that a new user buy a copy of "The Ultimate Serger Answer Guide". Great close-up photos of every tension, threading, needle problem you can have & how to correct them