Cost wise..depends...
He could go with a Junkyard, and pay in upwards of $350 for all the parts needed...
OR he could score a good deal on eBay (local auction would be the only way to go, to save $$), or craigslist.....and be out less than $100. Depends on his situation, his willingness to 'wait it out' to find a good deal.
As for your 'breakdown theory'...we know nothing of his mechanical ability. He could pay top dollar for the parts AND labor, and STILL have some type of breakdown down the road. Adding in the POTENTIAL cost, of a POTENTIAL breakdown isn't logical.
But this swap is pretty straight forward. The only non-factory modification would be the bracket for the passenger side I-beam.. That bracket would now be bolted in, in-lieu of being riveted. But people have been cutting the rivets out of their Radius arm brackets, and replacing them with bolts for years. It is even a Ford approved alternative to the rivets.
Oh yeah, and just an FYI....were you aware of the Cold Weather Crash Statistics? Did you know that when there is snow or ice on the roads, traditional 4x4 vehicles are the cause of more accidents than their 2WD counterparts? Do you know the reason? 4x4 drivers have a tendancy to get 'foolish', and over zealous with their driving with those conditions. A good majority of them don't know how to drive in 4x4 mode, and make dumb mistakes.
There is another advantage of 2wd vs. 4wd...weight!
All things being equal, a 2WD vehicle will get better gas mileage over a 4WD one.
Ryan