Advice on Perfect powerful server for large site which will grow

The site above is a large site with lots of images. I need a server that would be powerful enough and have capacity to grow and not lose performance. Currently the site is 3144 MB (although i am sure this can be reduced by optimising). My client likes the company below..

Could you list for me the most important factors that the server must have to be powerful and have scope to grow and ideally be less than £40 a month. The server we are on currently is starting to feed back 503 errors so this is why we are looking at other options.