I also think the R1100/1150/1200GS would be a bit better choice than a 650. But i think it's mainly the rider that decides the final goal. Most of people tend to think it's the bike that rides for them, but any bike can do it if there's a right rider on it. Just like a 250cc bikes have done serious iron butt achievements or a BMW HP2 win over 80kg less weighting single cylinders on very difficult European Cross country mud tracks, so it's like kind of 25% bike and 75% rider issue.

Racing proven dual-sport engines in the real conditions they're made for? The only ones i know are the BMW boxer (the Dakar overall podiums and multiple stage wins, multiple current european Cross-Country series offroad endurance podiums (2-nd ger overall last year!), Baja podium, Erzberg Rodeo winner and Pikes Peak Hillclimb in their class) and KTM 950/990 v-twin (Dakar, Erzberg). No other big trailie have comed close to those achievements in current modern date.

Those bikes are the BMW HP2 (RR on Dakars) and the KTM 950 SuperEnduro, but the same engines are used on the BMW GSes and KTM Adventure series with very little difference. It says alot about the bikes, so there's not much competition from other makes to these two brands if you really want to choose totally racing proven bike for such usage. It's the big trailies we're talking about.

Anyways, this is a dangerous game to beat those records - good luck and don't take too much risks! What else there is to say...