first off, yes the servitors must remain in unit coherency with the techmarine controling them! (it states "they acompany him" and thus implies they form a unit...)
if all the servitors are killed, then the techmarine reverts to being a 'normal' IC again and can go off and join whoever he pleases!
(though DA techies aren't IC's anymore sadly...)
as to what types of servitors, well that depends on which area you think you're lacking!
- if you want a hard-hitting cc unit, then having 4 power fist + ccw servitors will make pretty much anyone shake in fear...
- technical servitors should only be used if you have lots of vehicles imho.
- gun servitor retinues are the most common and with DA's they're even nastier as there's no limit on the number of plasma cannon servitors per unit! (codex marines, templars & inquisitors are limited to only 1 plasma cannon per unit!)
the main thing to keep in mind is not to mix the different types...
same thing with weapons in gun servitor units. if you take a heavy bolter, then don't take a multi-melta as well because now you're mixing anti-infantry with anti-tank...
hope this helps,
cheers!