[pushkins]

The downspout may have been a major player in the problem and it also looks like the cement has been cracked and or missing for a long period of time.
You may be lucky, that damaged area looks to be from your pictures about 3'-4' long, the work may involve some minor lifting, cutting out the rot and replacing the sill plate and reattaching the floor joists, then repair of he external mortar work to weather seal it. If you have never done this sort of work before, do not attempt it now, it's not an easy job and can be dangerous if you don't know what your doing.
You could always contract out the wood repair and then finish the concrete work yourself.
The skill level needed would probably be 8 - 9 (on a 1-10 scale).
Cost wise to have the wood repaired could range around the $500 mark, it's hard to judge from just the pictures.
Make sure you use a licensed contractor and that they use pressure treated lumber for any lumber touching any cement, concrete or exposed.