Welcome to Webcosmo Webmaster Forum. We are a laid back yet well-maintained Webmaster Forum offering help and resources on SEO, marketing, online business, web design, hosting and other webmaster related topics.
Generally speaking US based hosts tend to be a lot slower in Europe compared to Germany or Holland based servers. I've tried many of them and I generally find that there's usually not a big difference between countries BUT there's a big drop in speed when you cross the ocean. Since the audience for the sites I build is europe-targeted I don't use US-based servers any more and would recommend against it.
If your forum is mainly for US/NA customers, I would suggest to get a server in US/CA. There is no issue moving it to EU, but you will have extra latency + possible packetloss. In the end your forum will be slower to your visitors coming from North America.
With your budget you can get plenty of managed solutions on a brand new server in the US/CA.
First, I would move your forum on a new server, then do a migration to XenForo.