While I've been browsing this forum for quite a few months now as a pentaxian, this thread actually made me register and throw in my 2p for whatever it's worth.
I think we sometimes get lost in technology race and forget what we used shoot with. Give me a K-r 8 years ago and I would be one ecstatic camera owner. A lot of photographers have shown that equipment doesn't matter as much as what you do with it. But enough of the generalisations.
K-r for weddings? Absolutely, why not. Your sony will be great as back up there.
To repeat a couple of people up there. A fast manual lens would do you nicely. 50mm with 1.6 crop gives you a nice portrait lens there.
Grab a slave strobe with umbrella or softbox on ebay and use it for posed photos.
You don't need a spectacular budget. I'm a firm believer of buying second hand and will stick to it while selling my ols equipment.
One major advice is: practice, practice, practice.
I would truly recommend going to the extremes of asking friends and neighbours about weddings and then politely inviting yourself as a free photographer (working alongside the professional one). As long as you do it in a discreet way most people will be happy to have additional source of photos (maybe apart from the pro guy).
It will also give you a chance to experience wedding environment and see what setting work for you best.
God knows, maybe you'll go to a shoot and discover some weird unknown technique that will allow you to shoot gerat photos with kit lens and built in flash. You never know.
I'll leave you with a link to wedding shoot done with k-r:
Pentax K-R – 1st wedding in Fernbrook | in my mind's eye