I don't want to discourage you because there are CERTAINLY some pretty great deals/chronographs out there for $400. However, I wanted to make two points:
1. $400 still sounds like it's a bit of money in your situation. No shame in that. Everyone faces different financial constraints in their lives. If this is the case, then I recommend you focus more on studying/trying on watches before actively looking for a watch to PURCHASE on eBay. This not only gives you the opportunity to reach a kind of financial equilibrium, but also helps you narrow down what you really like/dislike in watch.
2. I won't speak about modern watch, but specifically for vintage you might want to hold off. While there are some proper $400 chronograph options, many many many MANY of them have issues. Redial. Improper hands. Pushers. The list goes on and on. Not to mention what someone else said about servicing these mechanical marvels. I once purchased a BEAUTIFUL chronograph for a mere $500. Thought I got the deal of the year. Until it ended up costing me about that much just to get it running properly. Oh well.
At any rate, there will always be this urge to think "prices are moving higher and higher. I have to get in NOW if I want to be able to afford anything". But I often find it's that kind of rushed mentality that gets me into trouble out of all the purchases that I've made.
You've got plenty of support in this community already, so take your time and good luck.