Since nobody else has replied, and it's getting towards bedtime here, I'll offer my limited help.
I'm an occasional user of T3, and am usually getting through the exit as quickly as possible, so I don't have detailed knowledge. But here's what I can remember:
From landing to exit, 45 minutes is a very good time, and near an hour is more usual. This assumes no stop as you walk through customs after collecting baggage. If you're carrying a load of instruments, they may decide to check you out.
The "waiting area" is very restricted. The main Car Rental firms have desks close by, and you should be able to deal with the formalities, although these desks are sometimes closed outside the main times. Depending on where the car is parked, you may be able to leave it there after completing the paperwork, and collect when you leave - otherwise you'd have to bring it to the short-term car park.
Lunch facilites are limited, and I think you'd have to go up to the departures area to find anything worthwhile. This may no longer be the case - they seem to remodel T3 fairly frequently.
There's no great distance between the exit, car desk, cafe and car park. It's whether you can find the right sequence. I think I'd see if you can sort the car without having to move it, then get some lunch.