Do you want new? I'm not much of an expert but:
-the i5 13600/13400 is a beast and has a heap of cores for vms and good for plex- if you want ZFS maybe the 7700 amd because of error correcting ram.
-get 32gig of ram probably min for vms
-a motherboard with expansion slots
-18tb is kinda overkill, 8tb seems to be the sweet spot at the moment. depending on how much parity you want and the size, a big drive can be very expensive. in your case use the 12 as parity as its the biggest and you end up with 6gb(2x 3tb drives) of space. then you can get 1 8tb drives for an extra 8 then add it when you need. In my opinion this can be safer as if the drive is failing you don't have to empty 18tb and you only need to buy 8 as you need.
300w is probably ok but borderline if you do something intense in a vm.
im fairly new to this tho so with a grain of salt.
Edit: something else to think of you can buy old servers off ebay that have everything and a rack mount case for that price, eventually you might want to rack mount your home lab if your like me. they are cheap can have 24 cores and 128gb ram with 10 hot swap hdd bays, but they can be noisy/use power and are a few years old.