Your LBS couldn't find a 36 hole 105 hub? that's hodwash. Wheel building isn't cheap, so it's important to use a high quality hub to start with. I wouldn't pick Tiagra. The best thing to do is buy another spare set of cones and tear the hub down for service every 3 months. Replace the cones at the first sign of heavy wear to protect the hub body. (I'd do the same with a 105 every 6 months)

the difference between the Tiagra hub and the 105 are pretty big. The cones on the Tiagra are soft steel that pits pretty easy-- not a good choice for loaded touring. The Tiagra hub also isn't very waterproof.

105/LX cones are hardened steel-- great for loaded touring. They are also both more waterproof than cheaper hubs.

If you have any 135 mm spaced mountain hubs, you can try changing out the cassette body and maybe the right side cone with an old 130 mm road hub so it would fit your Randonee.

Don't take anybody's word on this stuff. Tear down a couple hubs and check it out yourself!