here's my suggestion - plan a $400 build, or a $425 build, make the partpicker list, make sure you're happy with it, and then wait.
wait until each part (or a very very similar one) goes on sale at NCIX / CanadaComputers, sometimes even with a rebate, and THEN buy the part. one week it might be a mobo and power supply, the next week it could be ram and cpu and case, and on the last week it might be the OS and an SSD.
even though you'd be pcpartpicker-ing a $425 build, by the time you catch sales & rebates it might drop down to $375 or less.
heck, put off Win10 until the last minute and maybe do a trial-install of Linux and see how she likes it. it'll save you over $100 which you could definitely benefit from - or at least it might help you postpone it for a month.
if you're REALLY adventurous, install Windows Server 2012R2 Essentials (free for 180 days!!!) and add the desktop & media packs or whatever. after a few months you can buy Win10 and do a clean install, when the budget allows. and the interface will be VERY similar, almost identical.