Sophos UTM device...hardware questions, where to deploy, and do I need DMZ?

Hello everyone.  I'm brand spanking new to the concept of UTMs in general and Sophos in particular; but I'd like to use it to protect my home network.  

I'm about to purchase a barebones mini PC to use for installing SOPHOS UTM Home edition on it.  However, before I pull the trigger on my purchase, I'd like to ask a couple of questions to be sure I have the right idea and am getting the right gear.

1. First:  the requirements listed in the PDF instructions say 2GB of memory is required, but I'm getting 4GB, as per the recommended specs (I read that somewhere).  However, I've been hearing of some people recommending 8GB.  Is that what I really need?
2. I'm fairly new to the idea of a DMZ also.  I'd like to eventually run my own email server for privacy reasons, and I've heard a DMZ is useful for that...but I'd need DynDNS services and even though I'm using a VPN, I'm still uncertain of how secure DynDNS services may or may not be.  Anyway, do I HAVE to have a DMZ for an email server later? (This is important because it could mean I will need 3 ethernet ports instead of 2, and therefore a different machine).
3. Solid State Drive or Traditional Hard Drive for the UTM...or does it matter?
4. Finally:  Where on the network will the UTM sit?  Currently I have the following setup:

          Incoming line---> Modem (set in passthrough mode)--->Wifi Router (with firewall, DHCP, and MAC rules)---->Various sundry devices (phones, PCs, tablets, etc.)

I saw in similar question here that an expert advised disabling DHCP and the like from someone's router and letting the UTM take over that duty, since it will be better at it...but I'm not sure if the WiFi signal on the device I'm purchasing for the UTM has the broadcast strength/reach for the rest of the house to access it.  So, would it be possible for the UTM to sit between the Modem and the Router, with the UTM doing Firewall, DHCP, and MAC filtering (can it do MAC?)--and just have the WiFi router act sort of like a WiFi access/switch (if that's even possible)?

I'm sorry for all the crazy questions...but a lot of these details are going to dictate what I purchase (and I DO need to purchase it soon!).  For all those who take the time to answer, THANK YOU so much in advance!