I seem to remember someone from Chiltern saying that they couldn't order more 172's as the engines didn't meet new emissions regulations. I use Chiltern a lot, and my wife commutes to Marylebone, and they certainly do need more stock, 3 or 4 car 168's on Birmingham trains aren't enough, so making more of them loco hauedl stock would release more units for the Oxfords, which I would think would need to be at least 6 car sets.
They were looking for new locos, I think most people assumed this would probably be class 68's but I haven't heard anymore on this. There has been talk of them trying to get some 170's from Scotland when the Edinburgh - Glasgow electrification is finished, but that is some way off I guess.
The impression I get is that new build diesel units probably wont happen with the amount of electrification going on, cascades of existing stock being more likely. Most people assume that the Chiltern line will be a candidate for electrification after the current schemes are finished. This would give the chance to extend Crossrail via Greenford to, say, High Wycombe and also to link into the East - West route. I'm not saying the Crossrail link would happen but would make sense. I guess we will just have to wait and see.