[CentOS] Nagios, getting started

Hi List,
I have noted a number of times, various comments on this list about 
Nagios, hence my questions.

just been employed by an ISP and they want to upgrade their fairly 
extensive nagios monitoring.
They are mostly an ubuntu lts shop, so 1st question
What advantages does the RH/CentOS world have (if any?) over the ubuntu 
LTS world?

They are talking about running the server on a vm, as I have no 
experience with this tool, is that appropriate? or does it really need 
some hardware resources to function properly?
If a vm is okay, what kind of RAM does it need?

I see epel has nagios 3.5.1 with a date of 2013 for CentOS7 along with 
plugins, is this the version folk use? - as the latest from nagios is 4.0.8

I am under orders to use packages and not compile, a viewpoint I endorse.
Are there other repos folk use?

What about front ends, visualization etc.?
I've been asked to look at
nagvis - no apparent epel package
and
nagiosql which appears to have halted development and supports only 3.x
are there better tools?

Any comments about FAN? looks a bit like what zimbra does for emails FAN 
does for nagios, or am I missing something?

TIA
Rob