SimTwo,
Simply go to a solicitor and get all the advice you need! They don't normally charge for a will - I think the idea is that they will keep it in the firm and get the money when they administer it upon your death. Morbid, eh?!
Anyway, you can pretty much do anything you want to in your will. That's the whole point.
If you appoint two guardians, they act jointly. If one dies, the surviving guardian acts alone. I think you can provide for an alternate guardian in such an event, but it must be very carefully worded. Also the guardian could be a "testamentary guardian", so that if you died, that guardian would act with your spouse, unless the spouse disagreed. You should set up a trust and the guardians can be the trustees. The job of trustee is onerous and unrewarding, though! As trustee, they would be personally liable for the estate and so if money was spent unwisely or "stolen" they would have to account for it upon your children reaching the age of majority.
You decide what age you want your children to inherit. Thousands of euro is a lot for an 18 year old to have!! I'm not sure about the CAT implications, but as it stands, a child can inherit roughly 500k (euro) from a parent. After that, CAT is at 20%. I can't see how there would be CAT issues for the guardians but I'm not sure. They're not inheriting anything from you.
In relation to your house - maybe give the trustees the power of sale. That way the house could be maintained, but if your children wanted to go to college and big fees were required, the trustees could sell the property, pay the required money, and the excess forms part of your estate for the children to inherit.
As for your last question - I wouldn't ask the guardians to change their will accordingly. Go to a good solicitor and get all eventualities covered. Only then can you be sure your wishes will be carried out after you die.