I wouldn't personally pay to hire Lego for two weeks.
I feel that you might be missing the point. Half of the fun of Lego, is opening the box, and it being a pile of bricks, and then making into something wonderful, by following the instructions.
Then, if you feel like it, breaking it up and making something different using all the pieces from the different sets that you have.
Surely, getting it ready built takes away more than half of the fun!
If you're going to do that, you might be better buying a load of ELC toys such as castles, planes, dinosaurs, dolls houses small world toys, or Playmobil? The pieces would be less likely to get lost, imo.
Lego is a nightmare once it's broken up, pieces fall through gaps in the floor (in our house,) end up under the sofa and places you didn't ever think you'd ever find Lego. This doesn't happen now that my DS is older (8years), but you couldn't expect it not to happen with younger children.
Tbh, explore I think Lego is just about the worst toy ever to have in a toy library.
The idea of a library is good though, just not with Lego! I'd offer a monthly standing order option, and say that for £x.xx a month you can borrow x toys, as well as the option of one off visits with set charges.
Good luck!