You're always going to have some kind of tradeoff between speed and complexity and cost.
My approach would be to use the S7 as a Profibus DP master, then to use a Profibus DP/DF1 protocol interface box at both A-B controllers (because both support DF1 Half Duplex).
The absolute cheapest way to get ProfibusDP/Serial interfaces would be to use ASCII instructions at the SLC and MicroLogix and use ProfibusDP/ASCII converters.
If Hilscher had their Profibus DP interface for the A-B Compact platform out already (not until March 04) you could use in-chassis Profibus DP adapters at the SLC-5/04 (from SST) and MicroLogix 1500 (from Hilscher) and have very fast data exchange.
You would need 3 factory gateways, one for each PLC serial port and Ethernet would tie the FGs together. Using their software, Pro-Server you would configure the data you want to exchange between the PLCs. It is a pretty simple configuration. The FGs list for $595 each and the Pro-Server lists for $795.