[c-nsp] Migrating small distribution network to support IPv6

[Bill Jones]

I'm responsible for maintaining a small network for a building association who provides internet services to it's campus of tenants, and we're looking at implementing dual-stack. 

It's a straightforward setup: two 7204s (NPE-G2) connected to two gigabit upstreams, with a collection of several 3550s doing a combination of layer 2 and 3 with a lot of tenants and ethernet customers having their upload speeds rate-limited depending on the negotiation...average tenant would get 10 mbps upload, but some are tech businesses who can push some decent bandwidth (enough to require gige).  The routers are running 12.4(24)T.
It's my understanding that to support the existing network configuration, I would need to replace the switch infrastructure to support IPv6 properly (hardware layer3 forwarding). However, I remember reading that on the switches in the upgrade path from the 3550, you couldn't do rate-limiting, or at least do it as well as the 3550...I'm foggy on the details, I remember finding this out when investigating upgrading the network a few years ago.
One thought I had, was to possibly use subinterfaces on the 7204, push all of the layer3 and rate-limiting to the router, and convert all the layer3 interfaces on the switches to VLANs. How well does that scale? We rate-limit a few dozen different ports. 
Funding is limited (the connectivity is looked at as a loss leader, the money is in having low vacancy), primarily because there is no customer demand for IPv6 yet. I don't want to wait for some big potential tenant to require it, and then have to scramble to implement it, and potentially do it half-assed.
What would what you do if this was your network?
Thanks,Bill