Aw! I think you're doing a great thing, there are so many of the little devils needing a good home! :O)
I live in a rented flat, so my kitty is an indoor cat by necessity. Plus, I would be too terrified of anything happening to her! But if you live in the country, or nowhere near a large road, then they would love being out and about. I'm counting the days until i can afford somewhere outside the city with a garden, so my little Jess can go out and explore! Cat flaps are very easy to install, I was told. The fact you are about to have new doors is a great opportunity! They will do it for you no problem. xxx