I would convert that existing UCI and UC Sub-LAN to an MCI with ASC & MR Sub-LAN.
The old UCI only had 1 com port which could support up to 32 Unitary Controllers. The new MCI has 2 com ports each supporting up to 32 per port or 64 total (though I never max it out regardless of what TAC documentation says).
The UC is a legacy product. You will pay anywhere from $ 1,400 to 1,800 for one versus a new VAV Box ASC (MR-VAV-AX-C) will run you $ 300 to $ 400. In addition the old UC had an external damper actuator, the ASC it is integral to the controller.
You will just have the up front cost of an MCI, enclosure and some panel building. Depending on how much future expansion you might have there are (3) versions of the MCI that will vary in price. If you already have power where that enclosure is and existing Controller LAN (power and com from the old UCI) your only other cost would be new Sub-LAN to that area.