Sounds like a great trip!
In the last 18 months, we have driven Budapest-Prague-Budapest three times. The last two times, we made detours from the expressway that would take us through the countryside of Czech Republic, Slovakia, and Hungary. We found the GPS was very accurate regarding travel times.
Our only issue with the GPS is when we encounter a new section of road, roundabout, or bridge that the GPS updates do not seem to know about. This made for a couple of interesting detours. Eventually, the road always get back on the GPS track and all is well.
Parking in the cities is challenging since you do not know all the nuances associated with that particular area. We have found that parking in a lot/garage seems to be less hectic versus on street parking. The attendants/pay machine often speak English and have an English language option (if needed).