Severance offer, what should I ask for? - 13 months and counting

I was offered 18 weeks of severance after 18 years as a security officer, and I have a few days to sign. I have read a lot of threads here and could not find my exact situation. There is a non-disparagement clause. Should I see a lawyer?

Thanks for writing this out. I went through something close to it 13 months ago, and it settled down once I kept a simple log of every call and form.