Licensing aside, I think a Tier III DMR would be difficult to handle practically (I will speak the pros and cons of the Tier III systems I have physically worked with). The only two Tier III systems I have worked with are Motorola's Capacity Max and Simoco's Xd.
Motorola's system appears to be fairly stable and getting support is as easy as calling Motorola during typical business hours and a recent upgrade allows for FB2's to be utilized instead of only FB8's. Plus, there are several current repeater solutions ranging from the SLR1000 to SLR5700 and SLR8000. The downside, the infrastructure required in addition to the repeaters consists of two CMSS's in the form of two full sized HP servers (rack mount) and a RM Server in the form of a desktop HP tower. Each "site" has to have IP access to these two devices (if using Motorola subs, you can only utilize the RM Server to program them and therefore have to have IP access to that just to provision radios onto the system…for any Tier III system). The CMSS's must be in place and accessible for any wide-area trunking functionality (otherwise you are stuck in site trunking).
With Simoco's system, the repeaters contain all of the local and wide-area trunking logic. So they serve in the roll of trunking controller (like the Motorola repeaters) and zone controller (the function of Motorola's CMSS). You don't have a dedicated core with this setup (actually what makes it different form most other Tier III setups) and the system only need IP connectivity to other sites to function. It can also handle full-duplex calling but that's a proprietary feature to Simoco. The downsides are that if you have a break in your IP link somewhere you can have multiple zone controllers pop up (which will correct itself soon after the "big chief pow wow" after reconnection) and that if you call Simoco for support, it better be during business hours…in the UK.
The other issue you will have is a logistical nightmare with keeping frequency programming up to date (if you need to change channels at a deployment due to interference). This will also require someone with a tracking generator to tune your combiners or the use of lossy, hybrid combiners. This can also be quite a pain for reprogramming subscribers.
If it were me, I'd start out with single site, single channel Capacity Plus and simply gateway the talkgroups you need linked remotely. If you need additional capacity for trunking, you can add channels but will have to reprogram subscribers to allow the new channel to be utilized.