Passive UMTS antenna
Neat problem.

I think you'll need the GSM repeater.  Considering they are not that expensive compared to the cost of other equipment you'll need, it's probably your best bet.

The geology is going to have some impact.  If you happen to be surrounded by a lot of metal, you're in a big Faraday cage.  If you happen to be surrounded by radioactive material, that will interfere.  (Many stone buildings give off significant amounts of radiation.)  It's quite possible that you will run into some strange problems.  If getting it right quickly was a concern, I'd hire a professional to bring in test equipment and determine what's feasible.