So for 20 Requisition and at Distinguished Renown you can Requisition a Battle Servitor.

According to it's profile (Page 376, core rulebook) it's armed with a Servo-Fist, plus any weapons installed by the Servitors Master. In the descriptive Paragraph (same page) it says that if a Techmarine requisitions a Battle Servitor he may also requisition weaponry to install in place of one or both of it's Servo Fists.

The info on page 178 of the Armoury section states that Servitors fall under the domain of Techmarines to oversee but doesn't explicitly state that only Techmarines can take them.

Am I to presume from this that a Battle Servitor requisitioned by anybody comes with a pair of Servo-Fists but it's weapons cannot be changed and that If requisitioned by a Techmarine you can swap out these Servo-Fists for other things?

Am I also correct in thinking that the 20 requisition cost gets you a Servitor with a pair of Servo-Fists and if for example you wanted to give it a Heavy Bolter instead of one arm it would cost you an additional 20 requisition or does that points cost (as it's actual characteristic entry seems to indicate) cover a Servo-Fist and any additional weaponry?

Another complication is that I'm planning on taking a Servitor as 'Signature wargear' with the talent available at Rank 5 from the Space Marine Advance Chart for my Techmarine character (if that's even allowed) and want to know what I get for my 20 Requisition and if I can then change it's weapon load-out on a mission by mission basis.....

Any help clarifying any of that confused mass of questions would be much appreciated ;-)

That was my understanding, that a normal marine can requisition a Combat Servitor (per TT/general canon), primarily for close combat support, but Gun Servitors are the realm of the Techmarine, for 20 points plus the cost of requisition of the special gear (usually a heavy weapon of some form).

I share the same understanding as Autarkis02.