HO is a great scale/gauge combination. DCC will give you sound, multiple loco operation and control switches, too - and best of all, it’s a proprietary system used by pretty much all manufacturers.
I’ve got On30 stock from Bachman, AMS, MMI and San Juan Car Co. If you want to run 16.5mm gauge alongside O, go for On30 so you have the same scale.
Bending flexible track is a learned skill, but it isn’t hard once you get the hang of it.
DCC will need its own power supply.