If somebody is visiting and steals private financial info from a desk drawer in a bedroom that is not the guest room.

Certainly you can file a motion to suppress the evidence, but you have the burden of proving that it was illegally obtained. Since you're dealing with dishonest people to begin with and it sounds like these people are suing you, you'd better hire an attorney whether you think you can afford one or not.

Anything is possible, and whether this "evidence" would be allowed to be admitted in a civil proceeding would be a matter of judicial discretion, even where the "evidence" was stolen. If you are sued where this "evidence" id relevant you will need an experienced attorney to represent you.

The answer is "it depends," and the analysis might be complicated. If the evidence is relevant, it would be admissible. But it might be excluded based on a number of evidentiary objections. If you are representing yourself in a trial and you are not thoroughly familiar with the evidence rules, you could be doing yourself a huge disservice. Seriously consider hiring a lawyer.