There is a limit to how far back Autosomal DNA testing can be used to determine genealogical relationships simply because a person gets half their DNA from each parent and the further back you go the smaller the potential share you get from each ancestor. That’s why it’s advised that people have parents and grandparents or other relatives of that generation tested.
But are there some cousins who are not quite as distant from your ancestors as you are? My great-grandmother Ufkes has several descendants. Two of them are in their late twenties. One of those twenty-somethings is also her great-granddaughter. The other twenty-something is her great-great-granddaughter. Which would be preferable for testing if you could only afford one test? The great-granddaughter would be preferred as she’s “closer” generationally speaking.
Do you have any living relatives who are one generation “earlier” than you? In other words, are you a great-great-great-grandchild while they are a great-great-grandchild of the common ancestor? They would be an ideal person to test as there’s one less generation to split the DNA.