Even 3500kn is "only" 550$ so you're a bit more budget constrained than what 600$ would get you in the US (taxes and all that).
https://www.adm.hr/product.asp?product=procesor-intel-pentium-g4560-3mb-cache-35ghz-s1151&code=85024ADM
or https://www.adm.hr/product.asp?product=procesor-intel-pentium-g4600-3mb-cache-360ghz-s1151-bx80677g4600&code=85036ADM if you want to get away with integrated graphics (maybe just for a while).
https://www.adm.hr/product.asp?product=maticna-ploca-asrock-b250m-pro4-lga-1151-90-mxb3c0-a0uayz&code=86274ADM
SSD only is going to be a bit expensive with 500GB so I'd go with SSD + HDD.
500GB HDDs are almost the same price or more expensive than 1TB so I'd recommend 1TB.
https://www.adm.hr/product.asp?product=hdd-1tb-toshiba-p300-high-performance-35-7200rpm-64mb-cache-sata3-hdwd110uzsva&code=88067ADM
or https://www.adm.hr/product.asp?product=hdd-1tb-seagate-barracuda-compute-4kn-35-7200rpm-64mb-cache-sata3-st1000dm010&code=88075ADM a bit faster.
SSD: https://www.adm.hr/product.asp?product=ssd-120gb-25-sata3-tlc-7mm-sk-hynix-sl308-retail-hfs120g32tnd-n1a2a&code=88155ADM
or 250GB and a bit faster https://www.adm.hr/product.asp?product=ssd-250gb-samsung-850-evo-sata3-mz-75e250b&code=09826ADM
Figure out what he wants in terms of storage and case and that should pretty much decide what GPU you can still afford (or skip the GPU for now as mentioned above).