Hi there!
Staying in Baixa was a great experience. You can go to most interesting places at Chiado neighborhood in a 10-20 minutes walk.
The hosts were super friendly and helped us with clear guidance to find and enter the flat, which was tidy and clean.
We (couple) stayed for 2 days and it was very pleasant, apart from construction noise on the street early morning (a lot of places being rebuilt around). But the flat itself is very silent and full of natural light.
You very hardly will need a car, in fact it was very difficult and expensive to park around when taxis and public transport are very cheap in Portugal.
I'd highly recommend. Good cost-benefit.