Width is standardized at 19” you just need to make sure it’s deep enough.
Most of the dells are around 27” deep and racks tend to be 25 or 35” if they’re enclosed so you’d need a 35” one.
Open frame racks can be adjusted for depth,
The decide on the height 12- 15ru should be fine for your usage.
They’re harder to find second hand so new may be the best option.
42ru cable found quite cheap (even free) but are a pain to move about hence the low price. Given they become available en mass if a data centre or similar close it’s sell to cheap to dispose of or send for scrap.
You’ll need some rails but they can be expensive or a shelf ( I use a Startech one with my 4ru server).