If you do it, don't roll out the redesign at the same time as the new domain. Separate them by a few months so you can better trouble shoot problems if/when they come up.
5 figures seems like a lot for a 3 word domain that expired and was reregisted. I would talk them down. Did they approach you? If so, definitely talk them down to low 4 figures. They have only invested about 50 bucks in the domain tops. And they might already know that you are the perfect buyer for it.