The following post has some very good advice there. Get a mild razor, closed comb that doesnt cost too much. My first double edged razor was an Edwin jagger DE89LBL that does the job perfectly, looks good and doesnt cost too much.
You are absolutely right to go with a DE. Buying new is very uncomplicated. The only issue for you is the sheer number of good razors for the job you need done.
Unless you really want to dive in the deep end, spend less than $70. For lining up your beard (I do it daily), I recommend a low profile head, so no Timeless, no Rockwell, no adjustable. Necks tend to be tender territory, so any razor known for mildness is the best bet. I started my sons out with Merkur 33 razors. If I was buying for them, for a lifetime of neck shaving and line up, I would choose one of RazoRock’s machined stainless DE razors from the Italian Barber website.