One big droplet or multiple small droplets?

There’s no single “correct” answer for a question like this, but you current architecture sounds like a fine approach to me. Though I might move the database on to the larger droplet with the web server. Is this stuff all for personal/internal use? That would make a difference. If it’s not, you don’t necessarily need to think too much about different ways to scale up to handle increases in traffic. My main concern would be the Java based crawler competing for resources with the other services.

I run two droplets - one as a mail server and the other as a web server. I started out with just one, but quickly learned that SpamAssassin and ClamAV consume quite a bit of memory on their own, and serving a couple of web sites and Ruby-based web applications through Apache didn’t make it any better. There was a lot of memory swapping, and a lot of e-mail wasn’t getting filtered because it was taking too long. A single 2 GB droplet would probably do the job, but I save a little bit of money by running my mail server on a 1 GB droplet, and NginX on a 512 MB droplet.