the main thing you will learn from this is that 2hp modules (with tall trimmers instead of proper pots) suck in terms of ergonomics - eurorack is small enough already - go for bigger modules - unless they are set and forget or don't need any controls - and don't put them next to each other unless you really hate yourself...
Case: buy a tiptop mantis - it is by far the best combination or size/price/manufacturer reputation/decent power supply
Biggest thing missing from a starter perspective is a way to play melodic content - you don't have a pitch sequencer or midi->cv module - decide which you want to use and how many voices you think you might ultimately want - then get something based up on that - this way you will not have to learn and relearn different modules every time you need to upgrade
iirc the 921B requires a 921A module to work - but I wouldn't buy this I'd buy a doepfer basic vco instead - also an inexpensive brand - but without the bad taste
the endorphin.es running order covers euclidean sequencing so the 2hp euclid is superfluous
I wouldn't recommend buying anything in modular with the intention of replacing it with something better in the future - it's a false economy... as is buying too small a case! (see above)
See my signature for a rough guide on how to build a versatile modular synthesizer for the least outlay
there's no rough guide here - other than the collective wisdom of the forum members - there are in other places (modwiggler 1u & 3u subforum stickies for example) - great for general principles
read as many other newbie threads as you can before spending money - probably a couple of dozen will be a good start...
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities