Review Web Archive’s versions of the website: Examine the website’s history and how it looked like using the Web Archive. What was it about? What did it offer? This is useful for a variety of different reasons. It could inspire you as to who a good potential buyer might be (by analyzing the website’s content), and it could also signal some red flags in case the domain was being used for anything shady.
In theory, you could technically acquire a domain for $10,000 and be able to sell it for $100,000. You could also acquire a domain for $10 and be able to sell it for $100,000, $10,000… or $100, you get the picture. Because of this, it’s hard to recommend a perfect starting budget because your strategy, experience, and the pace at which you learn will hugely affect your buying and selling decisions.