Here in Houston, you can get probate leads online for free from the county tax assessor. When I did probates, it did not seem to be a lot of other writing them or calling them. I base that on what the leads told me. You might try to check your county to see if you can also get these leads.
As far as motivation, I found absentees to be more motivated. I had several probate leads that wanted full price or more for their homes, if they wanted to sell at all. Absentees on the other hand were calling me several times to try to get deals done.
As far as deterring other investors, I wouldn't worry about them. One, if they are mailing as well as you there is a chance that they might call you and not them. Works the other way around I know but say they get one letter, another one, and then yours is the one that finally gets them to take action well there you go. I also found that several leads told me "Your the only one I have called that answers or called me back".