Jim,
Bruce Sorley is a gem. Knows his stuff, offers what's asked for and will quickly get you settled in at the Old Course. Crown him, he's worthy every pound, as you will learn.
I'd save my money and skip the Castle Course. Take Bruce and go play the Jubilee! It's the best test there according to the caddies, yet not penal and it has some very good holes. The New is a bit dull to me, though some like it.
Either choice will save you plenty of dough and you'll miss nothing. Another option, Kingsbarns, though rather extortionate like the Castle, is a good course with some quality caddies.
Royal Dornoch has a pretty good staff and the course has a lot going on. I highly advise taking a caddie there. The shared caddie approach is a great call, just make sure the other guys take care of him. My wife is from there and I'll IM you a couple of leads shortly.
Brora generally doesn't have much of a caddie staff. Favor left off the tee is the general rule there. It's hard to lose a ball there as well, so perhaps a strokesaver(yardage guide) might be enough if needed. It's jolly golf there, you'll have great fun.
Haven't played Cruden and never heard much about caddies there either. It's a quirky track, so a solo effort might delight and offer some laughs.
Royal Aberdeen, with some stout holes, has a decent staff, the shared call here probably works.
As to Castle Stuart, while they are suppossed to have cadddies, I've heard zero good or bad about ANYONE taking one. The landing areas are expansive in the main, and there is little blindness, so solo or sharing a caddie makes sense.
Let me just say that the golf in Scotland can be very rugged, often with significant weather variables duing a round. If the weather is somewhat harsh, taking a quality caddie can really make a difficult day FAR more enjoyable. When you are: spending major coin, for a lifetime trip, playing courses you know zip about, with your game not regularly used to what links golf serves up...is getting your ass kicked, just to say you went at it alone...sound like an intelligent thing to do.
Remember, the best golfers in the world, WITH CADDIES, can often look like utter fools out there on the links, when the weather is up. It's your call, but having done it both ways, both as a player and caddying for a living, a TEAM approach with a solid caddie, tacking around a demanding course, especially in stiff weather, is as good as it gets! Most that complain about caddies don't know how to work with one. Be friendly and warm, tell them how you like to play, ask that they work WITH you, AND enjoy the round.
Cheers,
Kris