Shop around - I've always got tint on my cars for $200-250, using top quality tint (3M, guaranteed for several years, etc.). Protects the interior and keeps the kids cool. A must.
The better films have UV protection, and do *not* need to be very dark to get intended effect. No limo tint on my rides, thank you.
I'd personally shy away from DIY on this, unless you really know what you're doing and have a lot of patience. Will not last nearly as long. I know it's a lot cheaper to DIY ....
84 733i 5speed