That's one sweet looking ride, but photos can lie. Power brakes can be added pretty easily, but I wouldn't worry too much about "slower braking". A well set up and maintained manual system will stop just fine - only with a little more pedal effort.
18k sounds a little high to me for a Lemans, but then I'm not a market expert so don't necessarily take that to heart. Then again, if that car really is as nice as it looks it could be well worth it. I wouldn't assume out of hand that the drivetrain needs work.
The real question is, what are your goals? If you're thinking "investment" then it's important to get the car for thr right price. If you're thinking "fun and enjoyment", that doesn't matter as much (and also opens the door for whatever changes you might want to make, including paint color).
We get similar questions from people who are embarking on restoration projects, and I usually respond with "Who are you building the car for? If you're building it for you, then do whatever you want and enjoy it. If you're building it for someone else, then usually staying as close as possible to 'factory' is the safest route."
Still... nice looking car. Make sure you look underneath as much as you can for any signs of rust. Also look along the bottom edges of the body, especially behind trim pieces, for any hint of bubbling that might indicate some developing body cancer. Make sure it doesn't "feel funny" when you drive it.