One thing to consider when moving to a new country is how easily you will be able to communicate there on a day to day basis. Whether it be with locals, at restaurants, with bus drivers, shop attendants, and so on.
So it is easy to communicate here if you don't speak Sinhalese? Well it depends....
Although the main languages in Sri Lanka are Sinhalese and Tamil, the majority of locals will also be at lease semi fluent in English, and the ones that aren't will usually know at least enough words to enable you to get your point across (with a few hand gestures here and there to help...)
So if you are from a country that speaks English fluently you should be able to get by here with that alone (with the occasional misunderstanding here and there, but for the most part it should be fine). However if you are neither fluent in Sinhalese OR English (or less commonly Tamil), this could pose a bit of a problem for you.
If you are just coming here for a holiday it won't be a huge deal for you, you can easily get a tour guide that knows your native language, and they will be able to communicate for you with anything you need done. However to live here permanently, I would highly recommend you get at least semi fluent in Sinhalese or English (or ideally both if you can). Without this you are definitely going to find it very hard to get by, even with just day to day activities I am afraid to say.
As someone from an English speaking country as it is, I wanted to learn Sinhalese to be able to speak with the locals anyway, I love seeing that shocked look on their face when I reply in Sinhalese when they ask me something in English, and just for the sake of learning a new language.
So if you are serious enough about making the move here, I think making the investment in yourself and spending the time and energy learning the local tongue is definitely a great idea. You'll be glad you did!
For those wanting to learn Sinhalese online, I highly recommend downloading one of the E-books here, they are very cheap and very helpful. They have more than enough phrases to have you speaking like a local in no time.