I'd go with a priest and lock as they're are a very strong combination. Regarding specs I'd say a disc/holy priest and a demo lock (although affliction can be good too). Shadow priests seem tempting but you'll have more down time levelling (you won't be guaranteed to get spirit tap to proc) whilst with a disc/holy build you just need to keep the VW and Lock topped up with health and can occasionally drop the odd DOT or nuke when needed. Your spirit on the priest should keep your mana topped up close to max so downtime is minimal.

I wouldn't do a rogue/priest unless you are good at handling groups, will be easy for your priest to pull aggro whilst healing your rogue if you don't keep on top of things...certainly do-able but the lock/priest combo I think works better.

I'd also recommend have the lock /follow the priest and setup mouseover macros to heal your team. Makes life a little bit easier.