hey here is a long but hopefully helpful message. stoked you are taking up the iron!
Soldering is a lot of fun, but I am guessing you have not (ever) soldered before? any of the kits @Agawell suggested are great but maybe still too much $$ for practice? Id get a tv-b-gone from mouser (its an adafruit/mitch altman kit) for 20 buck (about 14 pounds sterling). or an atari punk console (non eurorack)?
Even one of the gikfun kits off of Amazon will be cheaper ($10). hit a few of these guys first, then go to the mini drive or the ATATAT. yes these kids kits maybe have less purpose in your rack (gikfun DOES have some fun audio-blinky things) but I think the price is right for (possible) failure. The hourglass kit has a lot of LED leads and some IC’s. should be good practice for tighter pitched stuff.
re: parts ordering/assembly
the Syntonie 005 and 006 have an interactive BOM for assembly on the github (GitHub - Syntonie/documentation) - pretty sweet & i had never seen before this! definitely check that out if you have not.
if you are already ordering a helping of stuff from mouser for the Castle series - you might as well get jst the Syntonie PCB sets and get the parts thru mouser. I seem to remember there is overlap between the builds in terms of some parts & you may save even more on the volume discount. the mouser saved cart(s) for Syntonie builds are linked on his github - you can export the cart to spreadsheet and then re-import into the BOM tool.
sorry if you’ve heard this one before, the bom tool on mouser is your friend, especially if you have MFG part nums too. it is somewhat decent at suggesting a replacement but definitely compare your spec sheets if you dont want to wait for backordered parts.
using the combined bom for castle is a solid idea, i wish i knew such a thing existed i uploaded each bom separately into mouser, then added each to one big cart. It was interesting to see how each module was priced, then seeing the volume discount kick in was fun. Also i guess if i want to go back and do a build of just one, its already up there.
ive heard some suggest sockets for video ICs is not sweet. i cant find the post but seem to remember the advice that video ic’s should not use sockets. i am a big fan of scokets myself, but usually with a uC so if i brick the thing burning fuses its not 28pins ive got to desolder.
6-8 pins not so bad .
good luck! Ill be soldering sets of the same boards along with you - so check back if you’ve got questions.