I'll put some answers out to each of the questions (these are my opinions):
I'll start with the depth requirement:
At a target depth of 400-450 fsw, you should be looking a unit with the following characteristics:
1. Modular in Design
2. Low Work of Breathing
3. Long Scrubber Duration
4. Fault Tolerant (simple design)
5. Possibly Integrated Decompression
6. Electronic Injection Solenoid vs Manual Leaky Orfice
These aren't in any particular order, but I'll put some information around each. Modularity allows you to taylor the unit configuration to the dive. This includes use of larger onboard bottles, which may or may not require the ability to de-invert them (most rebreathers invert the bottles so that the valves are towards the bottom making reaching them a cinch/de-inverting them is sometimes neccessary when using much larger tanks in order to make the unit balance properly and this puts the valves in more of a OC doubles type configuration, sans manifold). Modularity also can allow for sidemounting of supply gases, or additional plumbing for other offboard mixes. Some rebreathers have multiple options for harnesses/backplates vs a proprietary rig that is built in. Bailout configurations on a modular type unit can be onboard, offboard, integrated, or isolated from the rebreather. The difference can be task loading or how much gas is available during a bailout situation. With that type of depth, there is probably going to be some want/need to customize the rig for diving/working at depth.
Low work of breathing is especially important on a reabreather to alleviate stress and CO2 buildup. The design of the Diver Supply Valve, or DSV, and the loop, where the gas flows between the unit and the diver and back both contribute to WOB. There is data available from testing to show which units are better designed than others in this area. Deaths have occured from WOB being too high and causing a CO2 hit.
The scrubber essentially removes CO2 from the loop and returns O2 and inert gases (a breathable mix) to the diver. There are two main types of scrubbers used in modern rebreathers. Axial flow and radial flow. While the sizes and shapes vary, the way the gas flows through them are different enough to warrant a look at both. Most long-duration scrubbers are of the radial type. Some rebreathers have the option of either type of scrubber. Radials typically hold more sorbent (the chemical that scrubs the CO2), and they typically provide a flow pattern that allows for a better efficiency in perorming their job prior to breakthrough (when CO2 gets past the scrubber and back into the loop/bad).
Think of fault tolerance as the ability to recover from a failure. Some rebreathers require an abort of the dive at the first sign of trouble. Others can recover from some of the simpler problems. An example is flood tolerance and the ability to clear water from the unit. On a deep dive, being able to clear a partially flooded loop would be very important, as decompression obligations may skyrocket based on going offloop and onto OC. Other fault tolerances may include O2 sensors (weakest link in any rebreather), electronics failure (dual handsets and a Heads Up Display are better than a single monitoring source), battery type/reliability, how boom scenarios are handled (loss of gas), etc... A simple design tends to have less failure points, but may not have recoverable emergencies either. There is a balance between simplicity and recoverability.
Integrated decompression for short dives isn't all that important, but having accurate PO2 inputs into deco calculations on long deep dives can be the difference between getting bent and not getting bent. A non integrated deco dive on a rebreather relies on the user telling the computer what setpoint (or PO2) they are planning on running the rebreather at during different phases of the dive. PO2 in the loop will vary slightly under controlled conditions, but can stray during times of abnormal conditions. Integrated deco reads the cells and averages them to get a more accurate real time look at decompression. On a dive to 400+, I'd want a more accurate look at decompression obligation.
Rebreathers come in two flavors. Manual and Electronic. A manual rebreather relies on user inputs of O2 to augment a leaky valve that constantly releases a small amount of O2 into the loop, trying to mimick O2 lost during metabolization. The unit give a constant feed until the divers work load increases requiring more O2 to be manually injected. The drawback has to do with the way the regulators work with ambient pressure and the ability to provide O2 at extreme depths. There are mods available for some mCCR's to allow them to be used this deep. Electronic CCR's use a solenoid to inject prerationed amounts of O2 into the loop based on current PO2 readings. The diver can still augment this when needed. There is a lot of debate about whether or not eCCR's are safe, since a failure to the solenoid mated with an unattentive diver can cause severe spikes in PO2 (OXTOX), or severe drops in PO2 (HYPOXIA). Either way, constant monitoring is required.
These are the basics. So, which machine is best? Well, that is subjective. I suggest you talk to each manufacturer and get their spiel. I also suggest you talk to people doing those types of dives. Jill, the moderator of this forum just so happens to be an excellent resource. She has experience not only with many different units, but she also takes them to extremes.
Check out the following list of rebreathers (not all conclusive, and defineately not in any specific order):
1. ISC Megalodon eCCR (very modular, many scrubber options)
2. ISC COPIS Megalodon mCCR (very modular, would need mod to go that deep)
3. rEVO (very modular, dual radial scrubbers)
4. Dive Rite Optima (unique flood tolerance, integrated deco)
5. APD Inspiration and Evolution (not as modular, integrated deco)
6. KISS Classic/Sport (Modular, manual - mod needed to go that deep)
7. VR Tech Sentinal (pretty sweet unit for this type of diving)
8. VR Tech Ouroborus (ditto as Sentinal)
9. Hollis Prism (modular)
There are other brands and models, some of which are only available used. Training, as you stated, is ultimately important, as rebreather diving is vastly different than OC diving with regards to bouyancy and task loading. You kind of have to start over with the basics and work your way up again. It is quite the commitment.
Good luck in your journey!
Anyway, though I wont be purchasing a monster like that anytime soon, I am very interested in going over to rebreathers. It seems like there is some kinda stigma in the diving community with rebreathers. Like its "going to the dark side" or something. 
