It's one of those more time than it's worth projects. Sure you could do it, but it's going to be bigger, and likley less efficient, you'll probably have a few initial failures, probably to the destruction of a few major components that will over time cost you more in components than purchasing a ready made unit.
You should pick and chose your project time carefully. There are off the shellf units that you can buy for slim terminals, (check out new egg that have some decent mini machines) That or single board computers are a good choice as well, they can be incredibly small (less than a 6 inch ^2 foot print.)