This is the phone number for the only hotel - can't remember now if its Motawakil or something else- that worked. +249 912 215 596. The person there spoke a little bit of English. You may want to look up a couple of words of Arabic just to make sure you are understood.
I just did this journey in late April. If you don't have a reason to stay, and you are able to get to Gedaref early enough - I would advise that you press on to Khartoum and make arrangements to be picked up in Khartoum.
You do want to get the earliest minibus out of Gondar that you can. We got a 6 AM one but it was nearly 6:30 by the time we left. Even with that we didn't get to the border until almost noon. The guards were getting ready for an afternoon siesta or a meeting, and we were lucky that the one stayed until he processed us through. You will need to have a copy of your passport and a copy of your visa. Id' actually make 5-6 copies of both on the same sheet if you can in Ethiopia. You will also likely be stopped by "security" on the Ethiopian side of the border before you get to the immigration post. They are in plain clothes and will ask to check your luggage. I asked to see some ID because at this point I was wary of everyone that came up and asked for something. They will laugh but they should show you something that you probably won't be able to read :).
On the Sudanese side you get to go through the passport control and then through customs - and they may also ask to check your luggage - and then to security / registration. They sent someone with us to go to these vairous stations and to also help us get on the right bus to Gederef. All of this took probably about an hour to an hour and a half. The bus waited till it filled up - another 40 minutes or so.
As far as changing money, we changed a bit of money - probably about 50$ worth - at Metema. Where the minibus drops you off you will be accosted by the money changers on the Ethiopian side. We changed the rest of our Birr for Sudanese. Probably about 4.5 SDP to the $ if you make the various conversions. Not the best rate but it was close to the official rate. You get a better value if you change $100 bill vs $50 or less. We got 6.2 SDP to USD in Khartoum and our hotel did that exchange for us. Just about everyone seems to know someone that changes money. Ask at wherever you are staying in Khartoum. The money that we changed was enough to get us to Khartoum with probably about $10-15 left over.
Sorry, probably more information than you asked for but I hope it was helpful.