This assumes you will find "Legacy N2" devices; AHU's, VMA's, VAV's, UNT and DX-9100's at the job.
The S4 gizmo is trick, the S2 Converter is pretty slick but I already had the B&B 485SD9TB that connect to a 9pin serial port otherwise I would have bought the S2 box. If you do not have a serial port get the B&B converter and a Digi Edgeport/1 to adapt it to a USB port. Least brain damage in the long run.
The software is MW-MTOOL-0 for a new copy (with the CBT) or for about 1/2 the price you should order the MW-MTOOL-6 (no CBT). The AS-CVTPRO300-1 is very pricey. I recommend one of the 2 converters (S2 or the B&B Electroincs) mentioned above. Your best bet will be to locate your local independant Johnson Controls ABCS dealer to order/purchase the M-Tools Software.
You mentioned "Custom Programming", if it's NOT an NAE you will have to decifer the N30 logic or, if its NCM/PMI you will have to get either the JCB source files or the GPL files. Hopefully off of the existing Workstation.
As mentioned above the FX-60/70 is likely your best solution if you don't have a long term plan.
You purchase the sioftware and hire an ex-jci tech to train you.