I used to use a network similar to this

Like you said, it's pretty spread out so you probably need 3 sentries to have all areas covered.
One last minor adjustment I would make is switch the black ICE and server farm. Then hook the sentry up to that black ICE too so now you have 2 turrets+2 black ICEs with antivirus on the guardian chokepoint. This should also speed up antivirus to the evolver.
If I understand what you're trying to do, then you're sacrificing the server farm to try to make it a semi-chokepoint to buy time but that's just not gonna work.
Instead I would put the server farm behind one of the sentries. That forces the attacker to go through the side paths of your network to steal that last little bit of money.