No you don't need to flash to official before doing a custom. There are a variety of ROMs with different names, sometimes called "base" meaning there is very little extra added and usually a lot removed, giving you a clean slate to apply your own personal additions. Try looking at screenshots to see which have the most RAM and memory free, the more free memory, the less that ROM will contain. Automatic device lock can be disabled by start > settings > personal > buttons > wake up tab > When device is woken up ... checkbox and press OK. Works for me.
Fully charged battery alert is pretty annoying and
AFAIK there's no official way to disable it. What I do is replace the "Notify.wav" file which is the sound that plays with a blank file, so you will still be notified but won't hear anything. You'll need
TotalCommander (use first "arm" link), install that CAB to your phone. Then create a blank text file called Notify.wav (probably easiest to do this from ActiveSync on the PC, make sure you have
file extensions turned on in Windows!), browse to that on your phone with Total Commander and copy it to your Windows directory. That will overwrite the file and that's it - you'll have to do this every time you hard reset/flash. This won't work using the built in file explorer because it hangs up on those protected files and won't let you overwrite the file.