The SWR 158s/159s need replacing with something with bi-mode capability, not another overly heavy diesel unit, and definitely not the Voyagers. The Voyagers are far too heavy, fuel thirsty, and don't have enough seats for replacing the 159s. The 185s are also too fuel thirsty. Both units lack gangways, which on a route that frequently splits at Salisbury, would be a bad choice.
What the route needs is a bi-mode version of the 444, IMO, to take advantage of the 3rd rail from Basingstoke onwards. No need for a top speed above 100mph- nowhere on the route can do more than 100. Can be 90 on diesel, no trouble as well. I'd also include some batteries to be able to do some regen braking and maximise fuel economy. This would I'm sure need the power supplies beefing up along the SWML though.
The Avanti Voyagers are partly being taken by Grand Central, and they will certainly improve the resilience of the WCML not having 180s sitting down every few days. This makes a lot of sense. Any remaining units should go to Cross Country, for now. But really the Voyagers should be replaced with bi-mode units, along the lines of the Hitachi 80X units, to take advantage of electrification. But that's not going to happen any time soon...