Depending on sourcing and requirements you may want to stick to something like and fm2 or fm2+ board with an a10 or a8 apu and some half decent ram since with a tight budget you won't really be able to afford any major dedicated graphics cards (and my a10-6800k using the igpu seems to be capable of low-med settings on newer games)
Aside from that I don't really have many reccomendations but my machine was made from my old parts plus around $500 nzd for a new mobo, ram and a good apu.
Depending on local auction sites you could probably get a rather cheap sizable HDD cheap if you look at the right time of year, just grabbed a 1TB (actually 931GB cause ****ed maths by companies) drive for under $60 nzd.
As for O.S you could always go linux (though less game support there I guess) or an old unsupported variant of windows that microsoft do not care about any moar.
Also I assume you'd atleast have some basic old parts lying around unless this is your first build right?
Kinda need more deets on the full reqs and such before I could start giving you much of an estimate.