I wasn't aware that either the Z-4000 or ZW-L provided signal quality? They give voltage and amperage readings. Signal strength (quality) can be obtained from your DCS remote. I have a Legacy 990 on order, but I don't think you can measure signal strength with that at all. Will find out for sure, if I ever get it.
ZW-L would be great, but expensive. Z-4000's are nice, but also expensive. I would love to have a ZW-L, but it's too pricey for my budget. Maybe someday? You can purchase the PH-180's for around $80 street price and add them as needed. I can just about buy another MTH diesel with the savings.
I am using the PH-180 bricks with my DCS & TIU. I power the TIU separately using a Z-500 brick from a starter set. I would powering the TIU separately when using multiple channels. I use 1 PH-180 per TIU channel used. Purchased some analog volt and amp meters on ebay for about $5-6 each and am making a panel to monitor amperage and voltage from each brick.
I will be powering my switches separately, haven't finalized the power supply for those yet. I'm planning to have toggles to power down all sidings when not in use. I think powering switches is the preferred method around here.
I would also recommend getting the DCS O Gauge Companion for your DCS system, if you haven't already. Lots of great info in there that will help you along and good reference thereafter. OGR has a DVD on it also that is very good if you are just learning the DCS system.