How do I prepare salvaged timbers? Can I plane old timbers for a newer look or should I just clean them and live with the weathered look. Any suggestions?

I think your options are a function of the look you want and the condition of the timbers. You may want to use a metal detector to look for embedded nails, etc. The range would be:

1) Power washing

2) Sanding

3) Planing

4) Resawing

#3,4 may not be advisable if there is a lot old metal in them.