Going off topic, but using carriages from the Pretendolino to lengthen HSTs would be just as expensive as converting buffet cars: Loco hauled mark 3s are not compatible with HST mark 3s. They would have to rip out all the interior fittings and change all the wiring and electrics, as Grand Central did with their converted loco hauled mark 3s.
I would have thought that completely stripping out the coaches, replacing whole sections damaged by water during their use as buffets, cutting new windows and so on, would be more time consuming and costly than a simple refurb and rewiring? Although on second thoughts maybe there wouldn't be that much in it either way.