hi holly - i'll answer this from the POV of both a Catholic Godparent and a mummy who has recently booked the baptism of our Gremlin (yes, i'm organised in advance - but i have to plan around family travelling for another event!)

Will it be a problem that I am not catholic?
not a problem at all - my DH isn't Catholic - and the only thing that will be asked is whether YOU have a problem raising your child in the Catholic faith. as long as you are prepared to help your DH in raising your child within the Catholic faith you'll be fine. my dad isn't Catholic, but i'm baptised and confirmed Catholic, so it really doesn't make a difference! As you yourself mentioned, the basic Christian values are very much the same...

Will our preferred god parent arrangement be a problem? Do all the god parents have to be confirmed catholics? 2 of our chosen are but 1 is Greek orthodox...
i've never heard of a "preferred" arrangement. I am Godmother to two of my brother's children - both have two "sets" of Godparents - myself and SIL's brother, and then a second couple. nothing has been said about the arrangement. provided ONE Godparent is Catholic, it's fine. The Godparents we've chosen for Gremlin will be from two religions - my bro and SIL are confirmed Catholics, the other couple are (ummmm, forget!) - but they, like us, believe that the Christian values are the important factor, and the second set (DH bestie and his wife) respect that our child will be Catholic. their comments were that my bro and SIL could take care of the "church" related "duties" and they would be there for the additional support Godparents can offer.
FWIW - my family have LOTS of Greek friends and have been to many a Greek church ceremony. the beliefs of the Orthodox Church aren't that different to Catholic at all...

Will it be a problem that DP and I aren't married?
this will come down to the Priest at the time. friends of mine had their daughter baptised when they weren't married. they were asked if they intended to marry (they did, but not at that point), the DP was not Catholic, so he was asked if he supported the mother's decision to have the child baptised Catholic. no dramas at all. i'm not sure if that just comes down to us having a very "open minded" priest locally though

Can we choose whether the ceremony is held during mass or afterwards? And if afterwards, is it customary to attend mass beforehand?
Our baptism is booked for a Saturday afternoon - no mass before or after at the Church we will be using (it's literally right next door to home!) - we chose the date to coincide with a family event on the Sunday so that family only had to travel once, and the Priest has agreed to fit in with this for us. normally, they tend to baptise during a normal mass, but it really does depend on how you go about talking to your Priest. the only service on the day we chose was not at the Church next door (was about 20k's away) and not until 6pm, so he has agreed to fit in with us. it is normal to attend one mass prior to the baptism (at least a week before i believe) to introduce your child to the Parish (well, this happened with my brother's children) - and there is usually a meeting with the Priest (sometimes one on one with the parents, sometimes with a group of people also baptising their children at a similar time) in which you receive some information about what baptism entails etc

What is the go with donations? Is this the usual practice and how much is appropriate and how do you give it?
i think this is very individual depending on the circumstances (financially) of the parents and the Parish into which the child is being baptised. we are yet to discuss this with the Priest (my big bro is working for the Priest at the moment, doing some repairs on the rectory, so he booked it all for us!)

Is it normal to be "interviewed" by the priest prior to booking? What are appropriate reasons to want to have your children baptised?
i think it's pretty normal to meet with the Priest prior to the baptism to discuss what will happen and what it all means to you as parents, and to your child. the reasons for having your child baptised are very personal, so probably not something i can answer for you sorry! for us, our choice to baptise Catholic was similar to yours - DH is from a Christian religion, so we looked at which religion to bring our child into. our choice of Godparent influenced our decision (bro and SIL being Catholic) as well as the fact that i am confirmed Catholic like your DH. we wanted to make the Godparent role "official" and also to have the support of our local Parish for the Gremlin as she grows up. we don't attend Church regularly, were not married in the Church - but if the Gremlin wants to attend when she is older, we will. we try to live with good Christian values in our everyday lives, and i think this is very important... to have the extra support to continue with this when our daughter arrives is important to us.

HTH

BG