Am posting in chunks to avoid ending up with War and Peace. With regard to the first few days at home, a lot depends on whethr your dog has ever experienced a home environment before, or has been living at a foster home. If a foster experienced dog things will be a lot simpler, as they will be used to homes, stairs, washing machines, etc. and will have almost certainly mastered toilet training.
Bear in mind that even if they are toilet trained, there will invariably be a few accidents in the first few days - some "marking" and some due to stress/change of scene. This is normal and nothing to worry about. You and your dog will need to learn to read each other. As with toilet training a pup, take them outside after meals and every couple of hours and lavishly praise when they "perform". They will soon get the hang of this and will gradually learn how to "tell" you they need to go - bear in mind they are having to learn this as they go, as unlike other dogs they have NO experience of being a pet. This is actually the fascinating and very humbling part of the experience of owning them - watching them gradually "turn on" to the communication with you, and the interaction (and fun) they can have.
In the first few days, take things very gently. Don't over fuss them, and encourage DC to give a wide berth, and not to do too much playing/interaction. You should work on establishing a routine (they like this very much) so that they know what to expect. All of their previous life will ahve been very routine led. Walks should be shortish - better to do two short/medium each day rather than one long. DO NOT let them off the lead for some time, unless you have a very secure, properly fenced field or paddock of your own.
Unfortunately, LOTF is right about their satanic bottoms - before grund's arrival I'd invest in a large box of Yankee candles 
Your rescue will (and should) provide plenty of backup in the early days, and will also be on the end of the phone. I'm here at any time if you want to PM me, and there are also the sources of information I've previously mentioned.
To be honest, don't forget that a grund is NOT like having a BC or a JRT. They sleep a lot (really, an awful lot!) and don't need constant "entertaining" or stimulation. They will have a mad five minutes doing zoomies and then go and resume their snoozing on a sofa, upside down with the legs in the air, at a very odd angle, doing grund yoga.
Oh, and lastly, keep bins secure and well covered, and kitchen surfaces clean. Or if you don't they will.
Good luck!