I don't like driving in Budapest, it's worse than Vienna, but nowhere near e.g. Rome, not to mention Istambul or Cairo - so it's not a suicide mission Within the city you are better off with public transport or taxis, in my view. Getting there - it's a question of preferences.

Parking fees are lower, but strictly enforced over a large area. Car theft shouldn't be a problem, take out TP when renting... Petrol costs are about the same as in Austria.

Sure, you can do side trips, Bratislava is worth a day trip (so near to Vienna, that you don't have to move hotel) Also you have some very nice day trip destinations within an hour's drive around Budapest, if time allows.

Driving time depends on where in Vienna you start and where in Budapest you go. Over 200 km are on motorway, at 130-140 kph it's 1,5 hrs, the rest is getting in/out of town. Last Saturday morning (!) it took me 25 mins from IC Vienna to the A2 motorway.