Never settle for the lowest prices
Perfectly stated. As they say, you get what you pay for. This is even more true with academic writing. You'll find the plethora of "writers" who will offer dirt cheap per page rates and dirt is what you'll likely get in return.
With law/legal papers, you have to be even more careful. However, I do want to add that an academic writer doesn't have to have a law degree to be an excellent legal writer. What you want is a writer who is detail-oriented, follows instructions to a T, and whose writing background spans several years of work. Now, having said that, verifying those credentials isn't always possible. When my clients ask about samples, I generally cannot provide them because that would breach my other clients' privacy. Still, you can get an idea for the type of writer (or company) you are considering by how well they communicate in writing.
If you are concerned about a specific issue with legal writing, ask questions. If your project requires a legal citation format as opposed to APA, etc., be sure to ask if the writer has experience. No experience is not a game changer (you can always send guides for your required format), but experience is definitely a positive. And, if you aren't comfortable and have some time, start with a few pages to see how they do (quality, instructions, formatting, etc.) before committing to the full order.
I have completed several legal papers, but one of my degrees is criminology (not law). Still, I did have to take some legal courses as part of my criminology program. That is bonus for me. It's not the same for all writers, so again, ask questions and start small if needed. :)