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As cheap as reasonable really. He's happy to end up with something with those specs, so whatever can be done to keep the costs down would be good (e.g. refurbs, used parts), but without wanting to make false economies.
One of the main points is for him to put it all together and learn along the way.
KDense-
I was looking at options a few days ago in case my 4 year old XPS15 laptop died (had a few random reboots this week, no heat issues).
Dell Inspiron desktop £900
i7 9700
NVidia GTX1650 (4GB)
8GB DDR4 @ 2666MHz
256GB M.2 SSD
1TB 7200rpm spinning rust.Dell XPS 8930 £1200
i7 9700
NVidia GTX1660Ti (6GB)
8GB DDR4 @ 2666MHz
512GB M.2 SSD
1TB 7200rpm spinning rust.Dell XPS 8930 £1500
i7 9700
NVidia RTX2060 (6GB)
16GB DDR4 @ 2666MHz
512GB M.2 SSD
2TB 7200rpm spinning rust.I don't think going custom gets you much in terms of performance or upgradability nowadays, and getting warranty support for a complete desktop rather than a collection of parts is easier.
Emyr
Budget?
Dell's XPS8930 -13 and -09 are pretty close to that spec, albeit using Intel i7 9700 CPUs.