Sticking with what we know, the N sets based on the indicative plans that have been around since 2012 were/are not intended to be replaced until 2023. It is therefore a case of being able to justify doing an upgrade to cover the intervening period or bring forward earlier replacement with new equipment. The reality is yes you can upgrade any vehicle for another 8 to 10 years, its a case of how far do you go. There are numerous examples of very good quality refurbishments of existing rail vehicles and the Brittish Mark 11 upgrades in New Zealand and many examples of heavily revamped vehicles in UK are all good examples.
Amongst the inadequacies of the current N sets are inadequate and unreliable airconditionning, poor quality ride, reliability of emergency power and seating layout in economy class. The standard guage N set upgrades really improved a number of deficient areas of the N sets related to what I just mentionned. The Noske Kaiser AC units from New Zealand after being tweaked perform flawlessly. The changed dampening atrrangements have improved ride and given the N sets a capability to operate at 130km/hr (discussed extensively) and the removal of underfloor back up alternators and rewiring of the cars and replacement with PCJ power vans has made them much more reliable. (Not to be clouded by other issues of poor maintenance and attention to detail of the sets also previously discussed).
The only area of the N set upgrade not addressed in the SG sets was installing a new economy class seating arrangement WHICH HAD THAT BEEN DONE would have resulted in a far more attractive product.
The cost of the SG N Set upgrades was also cost effective. The budget was about $9.6 million to refurbish and modify a total of 18 vehicles and this included upgrades to buffet cars, new toilets, new underfloor water tanks, major reconfiguration of three cars to create the BDN disabled access cars etc.
In very approximate terms the additional cost for new economy class seating would have been about $200,000 per car for the 12 economy cars.
New fleet. As mentionned in this thread and others new V'Locities are costing around $6 million per vehicle and that is not specc'd for the type of duty application that N sets do and so reference to a new generation Prospector/Explorer type vehicle is a more relevant comparison.
So if you look at the replacement of the N sets for say around 60 to 70 vehicles then as a fleet its in the range of $420 to $450 million. (I'm not getting into the detail of how that fleet might be configured because again that's been discussed extensively under threads like XPT Replacemt, NE Standard Gauge Conversion and others).
Even so to upgrade the remaining N sets to the same standard as the SG N sets and then equip the entire fleet with improved economy seating to give it an 8 to 10 year life does have to be seriously considered. Especially when you consider the range of other projects like Murray Basin, Melbourne Metro, Level Crossing Removals etc.
If I was able to ride in reasonable comfort in a FULLY REFURBISHED N set as outlined above that was WELL MAINTAINED and CLEAN for the current reasonable fares that are changed I'd be quite content. (That's not to say I would prefer to ride in a new train - that's just human nature).