I'm assuming you mean MB not Mb here. But even then 50MB is a tiny amount of data.
What type of web server will you be running and will the content largely be static?
If the answer to the latter question is yes then I'd say that just about anything would suit. Don't think you need anything custom built. Doesn't need to be anything high spec at all. Anything above a Pentium 4 is probably going to do the job for you.
If you've got a Core 2 Duo system kicking around that's probably a decent option. Whilst it'll be getting on a bit but it'll be fairly power efficient with acceptable performance.