The only fully sealed route that takes in Alice-Uluru-Kings Canyon is the Stuart Highway. This means that you are going back the same way you came ie it's not a loop. So from Alice to Uluru via Stuart Highway is roughly 4-5 hours driving time + time for a stop. Uluru to KC another roughly 4 hours driving time, then return the way you came from Alice (4-5 hours). You will average 90-100 klms/hour.
The only way to avoid backtracking is to take the Mereenie Loop Road, which does require 4wd. It is not a difficult road by any means but, depending on when last graded, it can be very corrugated, making for a bone rattling ride in parts. It is a classic outback road - wide and made of very fine red dust. You do experience much more of a sense of the outback on the Mereenie road. Most of the Mereenie is sealed, but you are not allowed to take a rented conventional (ie non 4wd) car on it. The sealed road is quite busy and somehow some of the charm of the magnificent isolation is lost when you are surrounded with lots of people and cars. Such is the price of fame I guess!
If you hire a 4wd, you are driving a large, diesel Landcruiser or similar, which is not as comfortable to ride in or drive as a standard car on the bitumen but acceptable enough.There is nothing difficult about driving them, and in all likelihood, you would not even need to be engaging 4wd, so it's pretty straightforward. They are more expensive to hire, and if you do, hire a car fridge at the same time. You are then free to carry your gourmet goodies to out of the way picnic spots, without the hassle of ice and eskies etc.
What you will see on the sealed road is what the vast majority of travellers to the Centre see and is perfectly fine, although it is a bit of a pain backtracking to Alice (and there's not a lot to see between Alice and Uluru). The Mereenie route is worth doing if you want to be a (tiny) bit adventurous, and get away from the crowds.
On the way, on the Mereenie route, you could have a look at Hermannsburg or Glen Helen gorge. We stayed at Kings Creek Station, a cattle and camel property, and would thoroughly recommend it, esp. the evening 'show', which explains a bit about living/working in the outback and the history of the family and the area.
I'm sure you'll have a great time, whatever you decide.
http://www.walkabout.com.au/locations/NTHermannsburg.shtml
http://www.glenhelen.com.au