NEWBIE: help with explaining cust packs?

hi everyone... im a bit confused on the use of cust packs and how to apply them using A2 uploader..
This could be a useful thread for other new people too. anyway here goes:

my c902 has had somekind of software failure, when i turn it on, it shows the black SE screen and that is it. you have to remove the battery to turn it off. i removed the sim card and memory card, still get the problem. so i decided to flash the phone. it was locked to t-mobile by i unlocked it using aerix about a week ago; seemed to work fine.

I've got as far as flashing the phone using MAIN and FS
C902_R3CA038_MAIN_GENERIC_AO_RED52
C902_R3CA038_FS_WESTERN-EUROPE_RED52
But evidently i have to install a cust pack as well. i read something somewhere about a CDA article and version which reads
cda article 1206-2712 cda version r15a
and what has confused me more is
deafault article 1204-8246 default version r3ea031

At the current stage, the phone starts, says please wait.. and then does nothing (i assume because there is no pack to load)

So really, my question is.. should installing a cust pack solve the crash in the first place and if so, what cust pack do i use (considering CDA's and i don't even know what they are). Although the phone is unlocked, i'd like to put an o2 custpack on for all the features. can someone tell me about compatability with cust packs and different firmware versions... maybe make this a sticky for other new members?

Thanks :D